The heat flow data of a decomposable volatile liquid obtained by a microcalorimetric technique denote the combined effect of two competing processes, viz., vaporization and decomposition. A model, therefore, is needed to evaluate the true decomposition parameters such as onset temperature, peak temperature, and enthalpy of decomposition. Such an exercise was carried out for the highly volatile and decomposable liquids such as nitromethane and acrylonitrile, using high-pressure differential calorimetry (HPDSC). A model has been proposed further under conditions where vaporization was suppressed as in HPDSC. On the basis of the experimental data, a mass-transfer coefficient was derived at low pressures (where only vaporization took place). This was employed for the prediction of the decomposition parameters at higher pressures. The predicted data agreed well with the experimental results carried out at higher pressures. It is believed that the results are useful in reaction systems in plants where chemicals such as the ones considered here are subjected to higher pressures.