The dynamic Young's moduliEof molded films of a polyethylene and a polypropylene were found to decrease with increasing thicknesst. An upper limit ofEappears to exist when the film thickness is reduced to less than 1 mil. For thicker films,Eis inversely proportional tot. Finally, the modulus approaches a lower asymptotic value whentis in excess of 20 mil. The results can be represented quantitatively by a model in which the surface transcrystalline region and the bulk phase of the polymer respond in a parallel combination to the applied stress. The modulus of the transcrystalline region in each polymer is higher than that of the bulk phase. The thickness of a surface region is about 12 &mgr; in polypropylene and 15 &mgr; in polyethylene. The effects of thermal history and the nature of the molded surface were examined briefly.