We propose the ACE (Adaptive Coordinator Election) platform that elects and relocates a coordinator adaptively in P2P networks. In collaborative applications, a coordinator mediates synchronization, consistency, sequencing and delay difference. However, it is difficult to decide a coordinator in applications used in P2P networks because of some characteristics of network, e.g., network instability, and differences in physical networks and devices for participants. The ACE platform elects and relocates a coordinator dynamically according to environmental metrics obtained from participants. Each metric has a priority and weight to allow a coordinator to be chosen according to the purpose of applications. We implemented our platform using the JXTA framework and tested it. The results show the feasibility of adaptive coordinator relocation in P2P networks.