The diffusion constant of water through transcrystalline surface layers on isotactic polypropylene (i-PP) is reported. A suitable experimental technique was developed, based on sorption or desorption measurements. The accompanying change in sample weight is detected by the corresponding change in mechanical resonance frequency. The facility allows investigation of the diffusion current parallel to as well as perpendicular to the sample surface. Tests with commercial polyethylene films revealed that reproducibility and reliability are better than 6%. The samples were prepared as thin films in which the transcrystallinity was controlled by use of several substrate materials and different thermal conditions during solidification of the melt. The water diffusion constant of finely spherulitic i-PP turned out to be 15.9 mu m(2)/s; whereas the values for transcrystalline material amounted to 19.6 mu m(2)/s parallel and between 12 and 10 mu m(2)/s perpendicular to the film surface, depending on the actual supermolecular structure. These results are discussed with respect to the underlying transcrystalline texture and the texture of the amorphous percolation network.