A concise stratospheric model has been used in a Monte Carlo analysis of the propagation of reaction rate imprecisions through the calculation of an ozone perturbation due to the addition of chlorine. Two thousand Monte Carlo cases were run in which all 55 reaction rates were varied simultaneously and individually. Excellent convergence was obtained in the output distributions because the model is sensitive to the imprecisions in only about 10 reactions. For a 5.7 ppbv (parts per billion by volume) chlorine perturbation added to a 1.5 ppbv chlorine background the resultant 1&sgr; imprecision in the ozone perturbation is a factor of 1.35 on the high side and 1.69 on the low side. The corresponding 2&sgr; factors are 1.81 and 2.86. Results are also given for the imprecisions, due to reaction rates, in the ambient concentrations of stratospheric species.