AbstractThe great disarray in the measurements of the maximum efficiency of conservation of light energy in photosynthesis is an outstanding problem in the development of photosynthetic biotechnology. The short‐term measurements of the efficiency based on O2 release by suspensions of cells or chloroplasts have given minimum quantum demands between 4 and 12 hv/O2. The defect of the short term measurements is that the effects of growth conditions, which can alter the efficiency by a factor of 2, have been generally ignored. In contrast, the steady‐state growth method, which depends on measurement of the maximum growth yield in photosynthesis, ensures that growth and photosynthesis are normally coupled. This growth method indicates that the minimum quantum demand lies in the relatively narrow range of 5.3 to 8.6 hv/O2. The question of whether the minimum quantum demand is less or greater than 8 hv/O2 is a crucial test for present theoretical concepts of the conversion of radiant energy to chemical energy in photosynthesis. To obtain the definitive maximum value more rigorously controlled measurements of photosynthetic efficiency in growing organisms are essential.