Abstract. In recent years, a number of commercial peer-to-peer TV (P2PTV) applications have been launched. Yet, their mechanisms and characteristics are unknown. In this paper, we study SopCast, a typical proprietary P2PTV system. Treating SopCast as a black box, we perform a set of experiments that are suitable to analyze SopCast in depth. We attempt to disclose the SopCast protocol. The dynamic nature of the SopCast overlay, in terms of node degree, is also addressed in this paper. Our approaches in analyzing the SopCast mechanism and characterizing its topological properties reveal important design insights in SopCast, and may help to better understand similar P2PTV systems.