Poly(vinylbenzyl)acetylacetonato complex of cobalt is an effective and stable catalyst for the epoxidation of cycloalkenes with molecular oxygen under 25 degrees C with i-butyraldehyde as the sacrificial reductant. The acetylacetone ligand/Co mole ratio, the reaction time, and the strain and electronic nature of cycloalkenes all affect the conversions of alkenes and selectivities to the corresponding epoxides. The catalyst can be recycled at least seven times without loss of its activity.