The proliferation of sensor devices and services along with the advances in event processing brings many new opportunities as well as challenges. It is now possible to provide, analyze and react upon real-time, complex events about physical or social environments. When existing event services do not provide such complex events directly, an event service composition maybe required. However, it is difficult to determine which compositions best suit users' quality-of-service requirements. In this paper, we address this issue by first providing a quality-of-service aggregation schema for event service compositions and then developing a genetic algorithm to efficiently create optimal compositions