The objective of this work is to validate an expanded version of a recently developed reaction mechanism describing liquid-phase decomposition of RDX. The validation involves a comparison of experimental results obtained from confined rapid thermolysis at various set temperatures. In the experiments, the decomposition occurs in the liquid phase, which results in evolution of species into the gas phase. The spectral transmittances of the gas-phase species are measured using FTIR spectroscopy, and these spectra are processed to obtain the temporal behavior of the evolved species using the HITRAN data base. A species conservation model was developed to simulate the confined rapid thermolysis experiments. The model incorporates the detailed liquid-phase reaction mechanism. The rate parameters in the reaction mechanism were optimized by comparing the experimental and computational results. With the optimized parameters, the computational model reproduces the experimentally observed trends with reasonable accuracy. Some of the deviations can be explained by experimental uncertainty. Based on the use of the computational model, initiation of decomposition occurs by HONO elimination. The subsequent decomposition occurs via the pathway starting with HONO addition and followed by ring opening. The detailed reaction mechanism containing 321 species and 500 elementary reactions was reduced to 53 species and 56 reactions using a sensitivity analysis.