Cultivations of photoautotrophic microorganisms show a rather distinct behavior from the cultivations of microorganisms on chemical energy. This is caused by the fact that light energy is the limiting substrate. Light supply is characterized by a constant photon flux which cannot be stored within the culture broth. In this study batch cultivations of Chlorella sorokiniana were closely followed in a bench-scale biocalorimeter (1.63 L, O 11.6 cm) illuminated with red light emitting diodes. Calorimetry provided the rate of light energy storage inside biomass representing chemical energy. Normalized to the total input of light energy this yielded the photosynthetic efficiency. It was highest in the beginning of the batch cultures: 14% at a cell density of less than 1 mL cells L-1. Efficiency decreased to 5.4% as cell density increased further to 5 mL cells L-1 (=2.5 g L-1). Efficiency was highest at low cell density when light penetrates far inside the reactor. Our results support previous findings that the presence of an increasing dark zone leads to reduced photosynthetic efficiency. The same trend was detected using measurements of carbon dioxide and ammonium consumption. The carbon dioxide consumption rate was used to calculate the enthalpy increase in the culture broth during photoautotrophic growth. The maximum photosynthetic efficiency calculated from the enthalpy increase was higher than the one measured directly in the biocalorimeter: 16% as opposed to 14%. This discrepancy probably is related to a combination of factors which are discussed opening up possibilities for further improvement of this technique. (C) 2007 Elsevier B.V. All rights reserved.