Data are reported in the present work to add to our understanding of the effect of medium (drug solvent) on drug partition and diffusion. The medium used was ethanol-water solution of which the property can be changed by adjusting the ethanol concentration. The membranes used included poly(ethylene-co-vinylacetate) (EVAc), poly(ethylene-co-vinylalcohol) (EVAL) and Nylon-4, three membranes with distinctly different swelling characteristics. The model drugs were benzocaine, a hydrophobic drug and theophylline, a hydrophilic drug. The results indicate that the drug permeability through the polymeric membrane can be adjusted by varying the property of the medium. By using the solution-diffusion model, the effect of the medium on the drug permeability can be decomposed into two parts: the effect on the partition coefficient and the effect on the drug diffusivity. The interplay between partition and diffusion determines the dependence of the drug permeability on the property of the medium. The effect on the drug diffusivity can be well correlated with the membrane swelling characteristics, and the Yasuda model can give a reasonable quantitative description for the dependence of the drug diffusivity on the swelling ratio. For the hydrophobic drug benzocaine, the effect of the medium on the partition coefficient is mainly dominated by its effect on the drug solubility. On the other hand, for the hydrophilic drug theophylline, the effect of the medium on the partition coefficient is much more complicated.