Zidovudine-ethylcellulose microspheres were prepared by water-in-oil-in-oil double emulsion solvent diffusion method. Spherical free flowing microspheres having an entrapment efficiency of 32-54% were obtained. The effect of polymer-drug ratio, surfactant concentration for secondary emulsification process, volume of processing medium and stirring speed of secondary emulsification process was evaluated with respect to entrapment efficiency and in vitro drug release behaviors. Infrared spectroscopy and differential scanning calorimetric analysis confirmed the absence of any drug polymer interaction. The in vitro release profile could be altered significantly by changing various processing and formulation parameters to give a controlled release of drug from the microspheres. The in vitro release profiles from microspheres of different polymer-drug ratios were applied on various kinetic models. The best fit with the highest correlation coefficient was observed in higuchi model, indicating diffusion-controlled principle. The n value varies between 0.23-0.54 obtained from korsemeyer-peppas model confirmed that the mechanism of drug release was diffusion controlled.