Abstract Changes were made in the composition of plasticized gelatin gels, analogous to those used in the manufacture of soft shell capsules, and the effect of these changes on the diffusion of ethanol through the films was investigated. The substitution of glycerol by higher polyols such as xylitol, sorbitol and lycasin (hydrogenated corn syrup) caused a significant reduction in the diffusion process, as indicated by a reduction in the apparent diffusion co-efficient. When allowance was made for film moisture content, a 5–10-fold reduction in the apparent diffusion co-efficient was observed with the greatest reduction occurring in films having lower moisture contents. This reduction in apparent diffusion co-efficient is believed to be due to a ‘phobic’ interaction between ethanol and the higher polyols which increases the energy barrier to the diffusion process. A general case is postulated whereby any diffusion process will be modified by the inclusion of ‘diffusantophilic’ (increased rate of diffusion) or ‘diffusantophobic’ (reduced rate of diffusion) components in the barrier. The implications of the changes in the formulation of the films for the filling of soft shell capsules by the rotary-die method is discussed. Attempts to modify diffusion by inclusion of solid particles or liquid crystals in the gel structure were unsuccessful, as was the replacement of part of the gelatin by hydrolyzed gelatin.