The hydrogen-isotope composition of leaf water and organic matter was studied in C3 and C4 plants. C4 plants showed greater enrichment in leaf water during the day compared to C3 plants. The difference in isotopic composition between C3 and C4 plants is due to their different water use strategies. C3 plants discriminate more against deuterium during photosynthesis than C4 plants. This difference in isotopic composition is not solely explained by physical processes.