In this investigation, our aim was to evaluate the performance of a controlled release of malathion pesticide loaded in microspheres of pure polymers of poly(3-hydroxybutyrate) and poly(epsilon-caprolactone) (PCL) and of blends of these polymers with compositions of 70/30, 80/20, 90/10, 95/5, and 97/3 w/w. Microspheres were obtained by an emulsion evaporation method to load a model pesticide. Scanning electron microscopy and differential scanning calorimetry were used to characterize the effect of the pesticide on the morphology and particles size of the biodegradable microspheres. The interaction between the matrix and pesticide was determined by Fourier transform infrared spectroscopy. The test for controlled release was performed in an aqueous medium, and malathion was quantified by ultraviolet spectroscopy. The results show that the release of malathion was improved with the PCL content in the blends; this indicated the possibility of modulating the release of the pesticide in these biodegradable blends. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3419-3427, 2010