Phenomena associated with membrane permeability are important pharmaceutically in drug-transport modeling and in the design of effective drug delivery systems. In the present study, it was found that under certain predetermined conditions the steadystate transfusion of dimethylpolysiloxane membranes by p-aminoacetophenone from binary solvent mixtures wasessentially controlled by the thermodynamic activity of the compound in the applied phase. This study augments and supplements present theories concerning drug transport, and it sharply contrasts with previous work where "drug delivery" was maximized at the coincidence of complete solubility and saturation for a fixed ratio of drug and mixed solvent and varying ratio of solvents. Under the conditions of these experiments, specific effects of solvent on mass transport were limited to slight changes in apparent diffusivity, D'. The D' values were calculated from lag times obtained by the Daynes and Barrer extrapolation. Appli~iition of these concepts to some practical drugtransport situations is discussed. Keyphrases 0 Drug transport-effect of solvent and solubility. paminoacetophenone across dimethylpolysiloxane membranes 0 Membrane diffusion--thermodynamic activity of the diffusing compound in the applied phase a Topical drug availability-thermodynamic control Dimethylpolysiloxane membranes-diffusion of paminoacetophenone 0 p-Aminoacetophenone-diffusion across dimethylpolysiloxane membranes