Models are presented for the height distribution of various photochemically active gases in Venus' upper atmosphere. Attention is directed to the chemistry and vertical transport of odd hydrogen (H, OH, HO 2, H 2O 2), odd oxygen (O, O 3), free chlorine (Cl, ClO, ClOO, Cl 2), CO, O 2, H 2 and H 2O. Supply of O 2 may play a limiting role in the formation of a possible H 2SO 4 cloud on Venus. The supply rate is influenced by both chemical and dynamical processes in the stratosphere, and an analysis of recent spectroscopic data for O 2 implies a lower limit to the appropriate eddy coefficient of about 3 × 10 5 cm 2/sec. The abundances of thermospheric O and CO are determined largely by vertical mixing, and an analysis of Mariner 10 measurements of Venus' Lyman α airglow suggests that the eddy coefficient in the lower thermosphere may be as large as 5 × 10 7cm 2sec. The corresponding values for the mixing ratios of O and CO at the ionospheric peak are approximately 1 per cent. The Lyman α data could be reconciled with larger values for thermospheric O, and smaller values for the vertical eddy coefficient, if non-thermal loss processes were to play a dominant role in hydrogen escape, and if the corresponding flux were to exceed 10 7 atoms/cm 2/sec. A sink of this magnitude would imply major depletion of Venus' atmospheric water over geologic time, and would appear to require mixing ratios of H 2O in the lower atmosphere in excess of 4 × 10 −4. The extensive component to the Lyman α emission measured by Mariner 5 may be due to resonance scattering of sunlight by hot atoms formed by charge transfer with O +. The H scale height, therefore, may reflect the temperature of positive ions in Venus' topside ionosphere.