In this Letter, non-equilibrium dissociation rates in expanding gas flows are studied taking into account state-to-state vibrational-chemical kinetics. A comparison of the results obtained in the state-to-state approach with the ones based on the quasi-stationary two-temperature vibrational distributions for harmonic and anharmonic oscillators and on the thermal equilibrium one-temperature distribution is presented. The influence of vibrational energy transition probabilities and nozzle profiles on vibrational level populations and dissociation rates is also discussed. (C) 2003 Published by Elsevier B.V.