The present paper contributes to the classic thermodynamic analysis of photosynthesis. Photosynthesis is a complex endothermic process involving two kinds of matter; substance and radiation. This threefold study consists of. (1) an energy analysis (the energy conservation equation is developed to estimate the energy effects of the process); (2) entropy analysis (the changes of entropy are used to estimate the irreversibility of the component processes); and (3) exergy analysis (developed for thermodynamic evaluation of both kinds of matter). A simplified mathematical diffusive model of photosynthesis (controlled by diffusion of gases) is used to analyze key aspects of the problem. The corresponding terms of energy and exergy equations are compared and discussed. Photosynthesis is a highly irreversible process. In the calculation example the irreversibility loss is 97.4%. This increases slightly together with increasing temperatures of leaf and environment. On the contrary, the energy equation does not show any loss due to irreversibility. In the calculation example for the presented model, the exergy degree of perfection for photosynthesis is 2.61% and the energy degree of perfection is 35.4%. The rate of sugar production is 3.21 x 10(-9) kmol m(-2) s(-1). Heat transferred to the environment by radiation and convection is respectively 6.4% and 65.3%. The exergies of the radiant and convective heat are 0.003% and zero, respectively. The impacts on several output parameters (e.g. leaf temperature, sugar rate, degrees of perfection, etc.) were determined: the effect of varying input conditions (e.g. environmental temperature and humidity, leaf absorptivity, intensity of solar radiation, coefficient of convective heat transfer, and CO2 concentration). Some possibilities for further developing the exergy analysis are discussed. (c) 2007 Elsevier Ltd. All rights reserved.