Stress-deformation curves were obtained for all samples of simple polyvinyl chloride (PVC) compositions before and after aging. Elongation at yield point and elongation at break were determined from the curves. Before accelerated aging, a significant shift of the yield point toward higher stress, and a decrease of elongation at break was observed with decreasing plasticizer content. The elongation at the yield point was not affected by the content of plasticizer. The curves of aged samples indicated as increase in rigidity: the stretching force corresponding to the yield point was higher than for nonaged samples, but the elongation at the yield point was only slightly lower. The elongation at break decreased considerably after aging, but was affected less by the plasticizer content. The OMYA BSH filler did not affect the mechanical strength of PVC, kaolin reduced it somewhat. These results show that the elastic properties of PVC are not affected by thermal aging. However, in view of the fact that the viscous zone increases with plasticizer content, and that this effect is intensified by aging, it is advisable to reduce the plasticizer content as much as possible in order to obtain a higher elongation at break, in conformity with standard specifications. Since the mechanical properties at the point of rupture are of little practical importance, it is suggested that the specifications be modified accordingly. As it is done for polyethylene, the yield value rather than the elongation at break should be the basis for standard specifications.