The production of O by algae during photosynthesis at steady state depends on irradiance according to a classical equation for the activity of enzymes inhibited by high concentrations of substrate. The equation is derived from a kinetic analysis of plausible reactions between photons and plant pigments, and it predicts that the inhibition of oxygenic photosynthesis often observed at full sunlight is an intrinsic property of these reactions. The equation contains 3 parameters, which are all sensitive to temperature. It provides a mechanistic basis for analyzing adaptive responses of oxygenic photosynthesis, but a more complex theory will be required to account for the effect of irradiance on the assimilation of C during photosynthesis.