Gas permeability in poly(vinyl-p-isopropylbenzoate) (PVp-i-PrB) was determined by a time-lag method. The transport properties were discussed from comparison with the permeability data of other poly(vinyl esters), which were studied previously. All these polymers are structurally related, and the size of a side group or the position of its substituent was changed systematically. The isopropyl group of PVp-i-PrB is attached at the para position of a phenyl ring and is the largest in size. As a result gas diffusivity and therefore permeability were increased. The effect of the substituent on gas diffusivity was explained as it increases the interchain and intrachain distances. The discussion was supported from the comparison of the density data between PVp-i-PrB and other poly(vinyl esters). The diffusion coefficients of six glassy poly(vinyl esters) were correlated at their T-g and good correlations were shown to the free volume and its fraction. On the other hand, gas solubility was little affected by the change of an alkyl group on a phenyl ring. The solubility data of PVp-i-PrB and poly(vinyl benzoate) were shown to be clearly correlated with the critical properties of the penetrants.