The effect of leaf temperature, O2 and calculated O2/CO2 solubility ratio in the leaf on the quantum yield of photosynthesis was studied for the C4 species, Zea mays L., and the C3 species, Triticum aestivum L. Over a range of leaf temperatures of 16 to 35° C, the quantum yield of Z. mays was relatively constant and was similar under 1.5 and 21% O2, being ca. 0.059 mol CO2 mol(-1) quanta absorbed. Under 1.5% O2 and atmospheric levels of CO2, the quantum yield of T. aestivum was relatively constant (0.083 mol CO2 mol(-1) quanta absorbed) at leaf temperatures from 15 to 35° C. Atmospheric levels of O2 (21%) reduced the quantum yield of photosynthesis in T. aestivum and as leaf temperature increased, the quantum yield decreased from 0.062 at 15°C to 0.046 mol CO2 mol(-1) quanta absorbed at 35°C. Increasing temperature decreases the solubility of CO2 relatively more than the solubility of O2, resulting in an increased solubility ratio of O2/CO2. Experimentally manipulating the atmospheric levels of O2 or CO2 to maintain a near-constant solubility ratio of O2/CO2 at varying leaf temperatures largely prevented the temperature-dependent decrease in quantum yield in t. aestivum. Thus, the decrease in quantum yield with increasing leaf temperature in C3 species may be largely caused by a temperaturedependent change in the solubility ratio of O2/CO2.