A first comprehensive investigation has been made to unequivocally analyze the end groups of two vinyl polyperoxide polymers namely, poly(alpha-methylstyrene peroxide), and poly(methylmethacrylate peroxide), using matrix-assisted laser desorption ionization-time of flight-mass spectrometry, Fourier transform-infra red techniques and thermochemical calculations. In both the polymers, the end groups formed due to chain transfer reactions were found in large concentrations. Detail mechanism of the formation of end groups has been presented.