During transpiration, water vapour diffusing through the stomata moves through air that has no mean motion: its diffusive inward flux is balanced by a general outward mass flow. The effect is to accelerate outward diffusive flows (water vapour, CO2 in respiration) and retard inward diffusive flows (air, CO2 for assimilation) and alter the apparent ratio of diffusion coefficients. The magnitude of the effect is calculated theoretically, and estimated practically for an experiment in sugar-beet leaves. For this particular case the error in estimating mesophyll diffusive resistance is near 2 per cent, but for other conditions could be much larger. Estimated values of the apparent ratio of the diffusion coefficient of water vapour and carbon dioxide (true value = 1.59) ranged from –0.027 to 2.79, but could lie anywhere between –∞ and +∞.