Broad line magnetic resonance imaging has been used to profile water ingress into a series of well-characterized sandstone rock plugs at saturation levels below the vapor percolation threshold, The diffusion process has been found to be strongly anomalous, Rather, it is in quantitative agreement with a model based on parallel transport of water in the vapor phase and adsorbed on the pore surfaces with the vapor and pore surface water maintained in dynamic local equilibrium, Among other parameters, the analysis has yielded estimates of the surface water diffusion coefficients (mean 5 x 10(-6) cm(2) s(-1)) and of the critical depths of surface water below which the vapor pressure is substantially reduced by strong adsorption (mean 30 molecular layers), A temperature-dependent study of one plug has shown that the pore surface water diffusion is thermally activated and is governed by an Arrhenius rate law with an activation energy of 49 kJ mol(-1).