Sn films varying in thickness from 200 to 4500 Å have been prepared by vacuum deposition. The surface roughness has been held to a minimum and the grain size restricted to approximately 1000 Å by cooling the substrate to 80°K and depositing the film at an evaporation rate of 100 Å per sec. The film purities have been calculated from residual resistance data. The mean free paths are in the range of 1000 to 5000 Å. Critical fields for films in this thickness and purity range are reported. The London and Ginzburg‐Landau models have been used to calculate from experiment the penetration depth, which has been found to be in general agreement with Ittner's effective penetration depth concept. The temperature dependence of the penetration depth lies between the two temperature‐dependence predictions of the Bardeen, Cooper, Schrieffer (BCS) theory.