Photo-oxidation experiments oil propene-NOx-air and 1-butene-NOx-air mixtures were performed in the CSIRO indoor environmental chamber. These data were used to test the alkene sub-mechanisms from the Master Chemical Mechanism version 3.1 (MCM v3.1). A comparison of measured and modelled propene-NO, data showed that the mechanisms required the inclusion of O((3)p) + parent alkene reactions in order to adequately model the results over a wide range of VOC/NOx ratios. Sensitivity studies were performed oil propene-NOx and 1-butene-NOx simulations to determine the effect of uncertainties in the chamber parameters and key photolysis rates oil the O-3 and NOx profiles. The low VOC/NOx propene simulations were sensitive to the formaldehyde (HCHO) photolysis channel that produces HO2 radicals and to the presence of O(P-3) reactions. For the higher VOC/NOx propene simulations, the O-3, and NOx results were more sensitive to small changes in the initial HONO and NO2 concentrations. Crown Copyright (c) 2005 Published by Elsevier Ltd. All rights reserved.