The times of penetration of 1M sodium thiosulphate and D163 developer at full strength and diluted were measured through gelatin coatings of 4.6, 10.5 and 20 mu, dry thickness, with a thin underlayer of silver chloride emulsion as an indicator. The results show that the time of penetration is proportional to the product of the thickness of the swollen layer and the cube of the reciprocal of the proportion of water in the gel. For layers of gelatin of up to 20 mum, swollen to any extent of practical importance, the penetration is non-Fickian; the solute moves with constant velocity through the depth of the layer, presumably as a front of constant concentration gradient.