Copolyperoxides of indene and p-tert-butylstyrene of different compositions were synthesized by free-radical-initiated oxidative copolymerization. The compositions of the copolyperoxides, obtained from H-1 and C-13 NMR spectra, were used to calculate the reactivity ratios of the monomers. The reactivity ratios indicated a larger proportion of indene units in random placement in the copolyperoxides. Thermal-degradation studies by differential scanning calorimetry and electron-impact mass spectrometry supported alternating peroxide units in the copolymer backbone. The activation energy for thermal degradation suggested that the degradation was dependent on the dissociation of the peroxide (-O-O-) bonds in the backbone of the copolyperoxide chain. The flexibility of the copolyperoxides was examined in terms of the glass-transition temperature.