The effect of dicumyl peroxide (DCPO) concentration on the polymerization kinetics of a polysilazane system has been investigated through DSC analyses. DCPO acts as polymerization initiator by activating the polyaddition of vinyl groups. The overall heat of reaction is found to be a linear function of the DCPO concentration above 1.5 wt%, due to the addition of the heat of fully initiated polymerization and the heat of DCPO decomposition. Below 1.5 wt% of DCPO, the polymerisation of polysilazane is not fully initiated and only part of the reaction occurs. The resulting overall heat of reaction is therefore lower than that provided by the linear law valid for higher concentrations. A two-reaction kinetic model built on this interpretation leads to a satisfactory representation of the DSC analyses performed for each DCPO concentration between 0.1 and 20 wt%. POLYM. ENG. SCI., 58:859-869, 2018. (c) 2017 Society of Plastics Engineers