A three-dimensional model was constructed for studying the diffusion of CO2 molecules from air into stomatal air spaces and further into leaf mesophyll cells. The model covers two different phases, mesophyll and air, with appropriate boundary conditions. Sinks for CO2 molecules were obtained from a well-known biochemical photosynthesis model parametrized for Scots pine. The effects of the varying stomatal geometry on the CO2 flux and intercellular-air-space (IAS) and mesophyll concentrations were examined. The net CO2 flux is sensitive to the size of the IAS-mesophyll interface when the mesophyll transport coefficient is low, equalling the diffusivity of CO2 in water. The simulations have revealed an optimum between the volume of the assimilating mesophyll and the air-mesophyll interface. The solubility of CO2 molecules into water in the cell surfaces, which depends on temperature and pH, had a clear effect on the flux. Increasing pH from the value indicated by the physical solubility (pH reverse similar6) leads to an increase in the temperature maximum for CO2 flux into higher temperatures and to unrealistically high CO2 concentrates in the mesophyll. Copyright 1999 Academic Press