The relation between water content of detached leaves of Populus euramericana ’robusta‘ and the rate of photosynthesis at various light intensities and carbon dioxide concentrations of the air was investigated. A reduction of water content was obtained by simply depriving the leaf in the assimilation chamber from any water. It takes some time before photosynthesis reaches a steady state level. A piece of elastic plastic tube, slipped over the petiole, contained a small amount of water, just sufficient for this period. The total amount of water transpired, as calculated from the area of the recording of transpiration rate, should be equal to the loss of weight of the leaf + tube during the experiment. Both measurements agree very well. Differences between + 30 and + 55 rag were found on a total weight loss of about 800 mg in experiments which lasted more then 6 hours and in which the water content of the leaves diminished from 80 to 50%. The moment at which the water was used up, was marked by the well known Ivanov effect. From thereon the amount of water in the leaf diminished at a rate equal to the transpiration rate. At the end of the experiment the fresh and dry weight of the leaf was determined. From the fresh weight at the end of the experiment and the transpiration data, the course of the fresh weight and water content, since the beginning of the desiccation, were calculated. The relation between the water content and the degree of limitation of photosynthesis by diffusion naturally depends on the CO2-content; it has been measured at 0.3% and at 5% CO2 in the air. At 0.3 % CO2 the decline of light saturated photosynthesis occurs between 77 and 64% water content. The reproducibility of this S-shaped relation depends to some extent on the vigour of the plants.