Conventional analysis of High-Pressure DSC (HPDSC) data gives only approximate values for the onset temperature of decomposition and the decomposition kinetic parameters because the data are masked by vaporization effects. A model is proposed to calculate High-Pressure DSC heat flow data under conditions wherein both decomposition and vaporization occur simultaneously. The vaporization effects are modelled using a single mass transfer parameter. Experiments were carried out to show that the parameter, once evaluated from data at a slightly higher pressure, could be used to estimate vaporization effects at even higher pressures. A method is suggested for determining the true kinetic parameters of decomposition.