The polymeric organotin-oxometalates [(nBu(3)Sn)(2)MO4] (M = Mo, W) were used as catalysts in the liquid-phase epoxidation of cyclooctene at 35-75degreesC. Using 70% tert-butyl hydroperoxide (TBHP) as oxidant, the molybdenum(VI) compound is much more active than the tungsten(VI) analogue, yielding cyclooctene oxide as the only product, in 88% yield, after 24 h. The solid catalyst could be recycled without measurable loss of activity. For both catalysts, activity increased markedly by using 30% aqueous H2O2 instead of TBHP, with no detectable loss of selectivity. The best results were obtained by adding a noncoordinating solvent to give heterogeneous triphasic systems (aqueous H2O2, organic, and solid). For example, with [(nBu(3)Sn)(2)MoO4] and dichloromethane, cyclooctene oxide was obtained in quantitative yield within 6 h at room temperature. Experiments were also carried out using other substrates (cyclododecene, 1- and 2-octene). The catalyst precursors were fully characterized in the solid state, including powder XRD, FTIR, and Raman spectroscopy, MAS NMR (C-13, Sn-119), and EXAFS spectroscopy.