A quantum-mechanical theory for state-selected unimolecular decomposition is developed in terms of the cumulative effect of a succession of internal collisions. An application to the model decomposition of CO2 to CO(¹Î£⁺)+ O(¹D) is developed within the restrictions of a forced harmonic oscillator approximation for the internal scattering matrix. Preliminary results over a restricted energy range yield a distribution of lifetimes over two orders of magnitude, 0.6â 60 ps, with evidence of a statistically determined behaviour on average but with wide fluctuations and with significant quantum effects on the branching ratios to different product channels.