And? I wonder why he hid it from you? Could it be that you made it clear to him maybe not directly but in some off handed manner that being friends with an ex is a huge no-no in your books and you will punish and persecute him to the fullest extent of your abilities if he ever does so? Your question is not that unique - look under cheating and relationship advice and you will find that this happens a lot - one of the partners are still 'close' to an ex - not having sex or cheating, but hanging around, talking, texting, being 'buddies' and to hiding it out of fear of causing a problem. Our source of 'knowledge' about relationships and what to do with exes is pretty much limited to what happened to others that we know and TV shows. Unfortunately most of the folk you know who been there got their knowledge from movies and TV shows. We are faced with only 'two' outcomes (according to the stories) that the exes end it and never speak to one another again, or they get back together. The reality is that many people are not compatible as a couple, but can still function as good friends. Many figure this out during of due to their failed attempt at a relationship. Many of them part company and never see each other again because as a society we tend to teach that exes can't be friends or on friendly terms. Society also teaches that exes will, if on good terms with one another, have to end up falling love all over again. It appears there is no wiggle room at least according to the GP (General Population). Psychologists will tell you something very different. The problem you see is that he is hiding something from you and you suspect the worse. I guess there is no wiggle room in your world either since his being together with the ex on friendly terms distresses you. Like it or not he and his ex share things - they share that life that they had together, they share material stuff, they share friends, family, loved ones all of these things is what makes him him - or the person you know. Unless you two have discussed this in depth then the reason why he lied should be assumed to be that you made it known or hinted at a continued friendship with the ex is a bad thing. Such a dead set boundary will always be a throne in the side of a relationship, it leads to slinking around behind your back, leads to lies which lead to doubt/mistrust and even the dreaded thought that he is cheating. I would suggest you two communicate and meet in the middle - compromise. Allow him to visit/talk/text what ever with limitations - say instead of visiting alone at her house that you go with him. Say instead of spending 4 hours a day on the phone chatting with her he spends only 1 hour. Being open and honest means allowing him to have room to work with that trust and to be open with you. BTW my spouse had an ex who pops up every so often. My initial reaction to the first 'pop up' was a bit negative, however once i got to know the guy and after talking in depth with the spouse I realized that they fully knew and understood in a deep way just how so unmade for each other they really are. They are friendly, share interest in each others family even discuss 'problems' in their personal lives (within reason) like friends or best friends would do. Once it was out in the open and both understood that I don't mind their meeting and talking and even going over the parent's house together AS LONG as I am being told the true innocence of the relationship was made clear. The spousal unit and I have a very open and honest relationship. There is nothing you tell me that I do not tell the SU, there is nothing you can tell the SU that will not be told to me. We are 'one person' meaning we know each other pretty well and know what we are doing with the other is not around. This kind of blunt honestly and sharing is not easy to come by. Both parties have to be willing and able to discuss rationally what is on their mind, and more importantly bot parties have to be able to sit there and listen until the other is finished talking without butting in, without jumping to conclusions, without allowing their own thoughts paint an ugly picture. We have rules of engagement when it comes to our discussions, we do not use terms like: "Yeah, but...." "Yes, however...." "Ok, yet...." Meaning we don't just play lip service to hearing what the other said and then let our 'problem' take over. We also do the 'I feel ________ when you ________" Meaning for anything that causes a feeling we calmly (when possible) lay out how we feel when the other does or says something. In the case of the ex, I made it perfectly clear that I was worried, that I was scared, that I was angry, that I felt betrayed. After 4 years together both of you should be this open and honest and able and willing to hear the other one out and to calmly or rationally say 'I feel ________ when you ________" and more, you should be able to sit there and really think before replying when the other says that and validate the others feelings without being defensive or offensive. To be perfectly honest I did not reach this enlightenment one day walking down the street. I went through a couple of failed relationships and a more than just a few personal and couple therapy sessions to learn these things.