Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5 to 35 degrees C either at constant O(2) and CO(2) concentrations or where the O(2) and CO(2) simulated the concentrations in water equilibrated at each temperature with the same gaseous phase. At constant CO(2) (14 micromolar) and O(2) (0.34 millimolar), the oxygenase to carboxylase ratio remained constant at 0.21 between 5 and 25 degrees C but increased to 0.26 at 35 degrees C. At O(2) and CO(2) concentrations near those expected in water equilibrated with air (21% [v/v] O(2)) containing 300 mul/l CO(2) at the various temperatures, the ratio of oxygenase to carboxylase activity increased 2.2-fold between 15 and 35 degrees C. At CO(2) and O(2) concentrations expected in water in equilibrium with subatmospheric concentrations of CO(2) in air (21% [v/v] O(2), 210 mul/l CO(2)), the oxygenase to carboxylase ratio increased from 0.25 at 10 degrees C to 0.56 at 35 degrees C. Between 20 and 30 degrees C, the apparent Q(10) value for the oxygenase reaction was 1.78 and that for the carboxylase was 1.26. Hence, the different responses of photosynthesis and photorespiration to temperature are due more to changes in the relative solubilities of CO(2) and O(2) (the solubility ratio) than to changes in kinetic parameters of the reactions catalyzed by ribulose bisphosphate carboxylase.