In this study membrane permeation was measured from finely divided pure powder and saturated aqueous solutions of two test chemicals with low vapor pressure, methyl paraben and 4-cyanophenol, through silicone rubber (polydimethylsiloxane) using diffusion cells operated to insure the concentration was negligible at the interface between the membrane and the receptor solution. The steady-state flux from the pure powder was substantial and similar to that from a saturated aqueous solution, but smaller by an amount that was statistically significant. However, there was no statistically significant difference in the ratios of the fluxes from the pure powder and saturated water for NIP and CP (0.66 +/- 0.10 and 0.72 +/- 0.07, respectively), suggesting a physical rather than chemical cause for the different flux results for powder and saturated water. One explanation is that diffusion proceeds from the entire membrane surface for the saturated solution, but only from the membrane surface in direct contact with the pure powder for these nearly non-volatile chemicals. In contrast to these new results, an earlier study saw no difference in the rate or amount of absorption of 3- and 4-cyenophenol into silicone rubber membranes mounted on an attenuated total reflectance Fourier transform infrared (ATR-FTIR) crystal. The results from mathematical model simulations of non-volatile chemical sources covering only a fraction of the membrane surface in the diffusion cell and ATR-FTIR experimental systems suggest that the measurement variability was large enough to make the earlier study insensitive to differences in the powder and saturated solutions. From a comparison of the mathematical model simulations to the new diffusion cell results, we estimate that less than 5% of the membrane surface had direct contact with chemical in the powder experiments. (c) 2007 Elsevier B.V. All rights reserved.