This article explores the effects of proscription -the act of listing an armed group as a designated terrorist organisation -as an example of how international counterterrorist policy can impact on the possibility for third-party actors to engage with listed armed groups in the context of peace processes. From a peace practitioner's perspective, the article draws on findings from a series of workshops organised with high-level mediators, counterterrorism policy-makers and diplomats, and interviews with policy actors and non-governmental peace-building organisations. The article concludes that the listing of armed groups as terrorist organisations is having serious consequences on the ability of third-party actors to engage with armed groups. It has affected both of the preconditions of access and trust, thereby narrowing the possibilities for third parties to effectively understand groups, influence them, affect their strategic calculations and train them in conflict resolution. The global proscription regime appears to be eroding in practice, with state third-party actors choosing to opt out of international proscription regimes. Doubts about the legality of third-party engagement are creating a new selective pressure on what types of conflict resolution activities are possible.