The diffusion of water vapour through pulp and paper was investigated experimentally in a diffusion cell. The diffusion cell consisted of two chambers differing in relative humidity. Any given sample was placed as a banier between the two chambers. Two pulp qualities and eleven paper grades were included in the investigation. The diffusivities measured ranged from 2.1.10-8m2/s to 5.4.10-6m2/. Effective vapour diffusivity was found to correlate with the density of the sheet. For five of the paper or pulp grades measurements were performed a t different temperatures and relative humidities. In the range of relative humidities investigated, effective vapour diffusivity was found to be independent of relative humidity.