Antiplasticization can occur when small quantities of a known "plasticizer" have been blended into a glassy polymer. Commonly, the T-g of the polymer and its free volume decrease. However, the mechanical properties of the antiplasticized polymer are altered significantly, causing the polymer to become stiffer and more brittle. Experimental results from flexural tests of polystyrene/mineral oil blends conducted at room temperature showed that antiplasticization is molecular weight dependent, thus supporting a hypothesis that the phenomenon can be attributed to a chain-end effect. A high molecular weight polystyrene (M(W) = 270 000 D) exhibited plasticization only, whereas a low molecular weight (M(W) = 40 000 D) exhibited both antiplasticization and plasticization effects. The 40 000 MW sample showed a 2-fold increase in flexural moduli and flexural strengths as mineral oil concentration increased up to 6 vol %. These moduli and strengths decreased rapidly at higher concentrations of mineral oil. Positron annihilation spectroscopy (PAS) data showed a 10% decrease in fractional free volume up to 6% mineral oil. C-13 NMR experiments showed that there was no change in the polymer backbone dynamics during antiplasticization. H-1 NMR Goldman-Shen experiments showed that antiplasticization occurs when the average diameter of the mineral oil domains is less than the average size of the free volume voids. It was determined that one mineral oil molecule was associated with a polystyrene chain end during antiplasticization. These results are consistent with the hypothesis that antiplasticization is due to a decrease in fractional free volume at the chain ends.