Since their first deployments in international peace operations in the 60s, police forces, or UNPOL, have seen their responsibilities expand. They are now seen as a crucial pillar of local capacity building. Confronted with new environments, police officers need to become familiar with new local contingencies and customs. Yet, they also have to adapt to their international colleagues and learn to cooperate efficiently with them. This is a key and challenging component of the implementation of their mandate. Based on 36 interviews conducted with Canadian police officers that have been deployed in the United Nations Mission for the Stabilization in Haiti (MINUSTAH), we focus on the daily practices and strategies they adopt in order to guarantee efficient police cooperation. The heterogeneity of the international contingents, considered here through various national practices with regard to recruitment ; the heterogeneity of the policing practices, and finally the candidates’ motivations to participate in such missions generates a climate of distrust and frustration within the UNPOL contingent. This increases the transaction costs among international contingents, thus hindering cooperation and further complicating the application of the UN mandate.