Small parcels (120 g) of wet wheat were embedded in 550 g lots of dry wheat and observed to take 65 days at 22·5°C and more than 140 days at 5°C to reach equilibrium. A difference in moisture content of 0·5 per cent remained after 140 days at 22·5°C due to hysteresis. In a second experiment, 16 cm columns of Cappelle wheat were placed in closed-end test tubes in a constant atmosphere regulated by a much greater quantity of conditioned wheat. Diffusion coefficients for both desorption and adsorption calculated from changes in test tube weight were 2·4 × 10 −6 to 2·6 × 10 −6 cm 2 sec −1 at 5°C and 7·1 × 10 −6 to 8·9 × 10 −6 cm 2 sec −1 at 22·5°C, over the range 8–18 per cent moisture content (dry wt). Discrepancies between measured and calculated desorption curves at 22·5°C indicate that the diffusion coefficient decreases substantially at low moisture contents. Reasons are given for supposing that the diffusion coefficient also falls to a low value at high moisture contents, though this was not explored in the present experiments. Moisture content profiles have been measured at periods up to 409 days when equilibrium still had not been attained at either 5 or 22·5°C. Calculated moisture changes in a large dry bulk exposed at the surface to a high relative humidity, and in a spherical damp patch immersed in a large dry bulk, indicate that moisture changes due to diffusion under practical conditions are extremely slow.