Theophylline was microencapsulated in composite whey protein-based wall systems containing different proportions of dispersed apolar filler, anhydrous milkfat. Wall emulsions exhibited uni-modal particle size distribution and had a mean particle size of 0.36-0.38 mum. Microcapsules were cross-linked by glutaraldehyde-saturated toluene via an organic phase. Spherical microcapsules ranging in diameter from 150 to larger than 700 mum were obtained and exhibited some surface cracks that could be attributed to the fragile nature of a peripheral, highly cross-linked 'shell' layer around the capsules. Core content ranged from 46.9-56.6% (w/w) and filler content ranged from 12.0-33.4% (w/w). Core and filler retention during microencapsulation ranged from 84.9-96.9% and from 85.1-89.6%, respectively. Core retention was proportionally related to the proportion of filler embedded in the wall matrix. Core release into SGF and SIF was affected by microcapsule size, type of dissolution medium and wall composition. Rate of core release was inversely proportional to filler content of the wall matrix. This could be attributed to effects of filler content on diffusion through the wall matrix and probably on swelling properties of microcapsules. Results indicated that incorporation of apolar filler in wall matrix of whey protein-based capsules provided the means to enhance retention of a water-soluble core during the microencapsulation process and to decrease the rate of core release into aqueous dissolution media.