The surface geometry of Vycor porous glass was examined by using the relative permeability (P-R) technique in a H2O/He system. At a water relative pressure of similar to 0.3, a maximum is observed on the P-R curve. This maximum is explained in terms of geometrical changes taking place due to the deposition of a water film on the internal Vycor surface. When the sample is dry, diffusion in a fractal system is found to be more appropriate in describing the process. When the sample is wet, the diffusion process obeys classical Knudsen theory for smooth cylinders.