It was hypothesized that since sub-stomatal carbon dioxide concentrations are often saturating to photosynthesis at ambient external concentrations in C4 plants at high light, photosynthesis might be insensitive to partial stomatal closure caused by large leaf-air water vapor pressure difference. The response of stomatal conductance and photosynthesis at high irradiance to vapor pressure difference was determined under uniform conditions in C4 plants grown under controlled conditions, and outdoors. In several cases, photosynthesis was less sensitive to stomatal closure than it would have been if photosynthesis had a linear response to sub-stomatal carbon dioxide concentration. No change in photosynthesis at up to 25 mbar vapor pressure difference was demonstrated in the C4 species Portulaca oleracea and Amaranthus hypochondriacus, despite reductions in stomatal conductance of 32 and 17%, respectively. Sensitivity of photosynthesis to leaf-air vapor pressure difference was found to depend on the species and on the growth conditions.