An analysis of the kinetics of simultaneous photosynthesis and photorespiration at the end of a diffusion path is applied to observed net photosynthetic rate as a function of O2 and CO2 concentrations. The data of Ku and Edwards (Plant Physiol. 59: 991-999, 1977) from wheat (Triticum aestivum L.) are analyzed in detail. Ku and Edwards, using an analysis that ignored diffusion resistance between the intercellular air space and fixation site, the competitive effect of CO2 on photorespiration, and the actual concentrations of gases at the fixation site, concluded that: (a) the affinity coefficient of the leaf for CO2 was approximately 3.5 to 5 micromolar; (b) this affinity coefficient is independent of temperature between 25 and 35 C; (c) the effect of O2 was independent of temperature over this range; and (d) competition between CO2 and O2 is responsible for the major share of CO2 loss from photosynthesis due to photorespiration. They suggest that using gas concentrations calculated as equilibrium values in the liquid phases is very important in reaching these conclusions. By applying a more complete analysis to their data which includes diffusion in the cell, it is concluded that: (a) the affinity coefficient of the leaf for CO2 is 0.1 to 1.1 micromolar; (b) the temperature dependence of this affinity coefficient cannot be determined from existing data, but there is no evidence to refute independent temperature effect on the two functions of ribulose-1,5-bisphosphate carboxylase-oxygenase being important in the regulation of whole leaf net photosynthesis; and (c) the competitive interplay of CO2 and O2 at ribulose-1,5-bisphosphate carboxylases may under certain conditions lead to a stimulation of fixation by the Calvin cycle because of photorespiration. These conclusions are reached whether CO2 and O2 are expressed as dissolved concentrations or as gas concentrations in the intercellular air space. The relative merits of these two expressions of concentration are discussed.