A model is presented for the distribution of photochemically active gases in and above the upper cloud layer of Venus. Attention is focused on reproducing the experimental results of the Pioneer Venus (PV) mission, especially the ultraviolet spectrometer results for the cloud top SO2 distribution. The reconciliation of these results with the photochemical oxidation and hydration of SO2 into sulfuric acid aerosol puts limits on the vertical mixing rates in this region, with a value of K around 2¿104 cm2 s-1 being preferred. A model with SO2 being the major sulfur bearings gas is consistent with the PV measurements. The measured SO2 is more than ample to produce the observed H2SO4 in the upper cloud region and balance the sulfur budget in this region. The ClOx chemistry above the clouds and the relatively large SO2 concentration in the cloud top region suppresses the odd hydrogen density and thus inhibits the catalyzed recombination of O2 with CO to form CO2. In conjunction with the slow eddy mixing, this leads to O2 mixing ratios at the cloud tops that are much larger than earth-based measurements; however, the major features of our sulfur model are not dependent on the O2 abundance.