A new technique in which ethylcellulose microcapsules containing theophylline (a water-soluble drug), prepared using the O/W emulsion non-solvent addition method, was developed. Toluene-cyclohexane was chosen as the solvent-nonsolvent system. The effects of four process variables, polymer concentration, species and concentration of emulsifier, and core to wall ratio, on the micromeritic properties and release behaviour of microcapsules were investigated. The results indicated that theophylline can be microencapsulated with a high yield (low drug loss) by using the O/W emulsion non-solvent addition method with the toluene-cyclohexane system. The particle size and drug content of the microcapsules were influenced by these process variables. The morphology of microcapsules was also affected by the core to wall ratio. The release pattern of the microcapsules was found to have similar properties to the release of a drug from a spherical homogeneous matrix. The effective diffusion coefficient increased with increasing core to wall ratio.