Thin films of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) have been subjected to transient pyrolysis using a pulsed CO2 laser in order to determine details of the thermal decomposition mechanism under conditions that simulate a thermal explosion. The first step, scission of an N-N bond, leads to formation of N2O4. The product is trapped in the solid film by rapid quenching to 77 K following the pyrolysis pulse and subsequently detected by transmission FTIR spectroscopy of the film. Product yield measurements show that 1.9 +/- 0.2 RDX molecules are destroyed for every N2O4 molecule detected in the films. Crossover experiments conducted on isotopically labeled samples containing both unlabeled and fully labeled RDX-N-15(6) show that the N2O4 product consists of a statistical mixture of (N2O4)-N-14,14, (N2O4)-N-14,15, and (N2O4)-N-15,15 isotopomers. These results show that both halves of the dimer arise from separate RDX parent molecules and that explosive decomposition of RDX involves loss of only a single NO2 molecule.