A method is described for the accurate measurement of the apparent resistance to water loss from the mesophyll cell walls of plant leaves (rw), and for studying the mechanism underlying this resistance. The method for distinguishing possible mechanisms involves a comparison of the calculated values of rw at different rates of evaporation. The value of rw remained below 50 s m−1 at relative water contents greater than 11 ± 3% and 7 ± 2% for Pelargonium hortorum Bailey and Vicia faba L. respectively. Therefore rw is relatively insignificant at normal physiological water contents in these species. When rw did increase it was not sensitive to evaporation rate, suggesting that a lowering of the vapour pressure at the evaporating sites was not involved. This contrasted with the results for cellulose filter paper, where rw was more sensitive to evaporative flux.