The mechanism of the gas phase unimolecular decomposition of hexahydro-1,3,5,-trinitro-1,3,5,-triazine (RDX) has been investigated using first principles gradient-corrected density functional theory. We have calculated the potential energy profile for two previously suggested dissociation channels: (I) N-NO2 bond rupture, and (II) concerted ring fission to three methylenenitramine molecules. The activation barriers for channels I and II are predicted to be 34.2 and 52.5 kcal/mol at the B-PW91/cc-pVDZ level, respectively. We have performed a simple transition state theory analysis which indicates that the prefactors for channel I and II are roughly 7 x 10(17) and 1 x 10(17) s(-1), respectively. Thus, our results suggest that path I is the dominant channel in gas phase thermal RDX decomposition.