The sodium tetraperoxo niobate, Na-3[Nb(O-2)(4)] . 13H(2)O (1) was prepared by fusing Nb2O5 with NaOH followed by reacting with aqueous H2O2 and characterised by single-crystal X-ray analysis and TGA. 1 was tested for the epoxidation of cyclohexene and oxidation of alcohols with hydrogen peroxide as a homogeneous catalyst. The [Nb(O-2)(4)](3-) anion was found to catalyse the epoxidation of cyclohexene with 30% aqueous H2O2 in homogeneous solution giving cyclohexene oxide in a 73% selectivity and 7% cyclohexene conversion at 45 degrees C. It also showed some catalytic activity for the homogeneous oxidation of alcohols (ethanol, isopropanol, 1- and 2-butanol) with H2O2. The [Nb(O-2)(4)](3-) anion immobilised on a quaternary ammonium polystyrene resin was much less active in the above reactions than the homogeneous catalyst.