The apparent diffusion constant, K, derived from profiles of radioactivity during labelled sucrose translocation, is temperature-dependent with a Q10 of 2·9 in Pteridium. This value is similar to those obtained by other authors for mass transfer rates. The behaviour of K with temperature supports a translocation mechanism of the ‘activated-diffusion’ type and is consistent with the Canny and Phillips translocation model which also suggests a basis for the 25–30°C temperature optimum in translocation.