Mono-transition-metal-substituted phosphomolybdates, PMo11M (M = Co, Mn, Ni), were synthesized and characterized by various physicochemical techniques. They were efficiently used for liquid-phase nonsolvent oxidation of styrene under mild reaction conditions. It was found that all of the catalysts are efficient in achieving higher conversions with high turnover numbers and are selective toward benzaldehyde. A detailed kinetic study was also carried out. Although all of the catalysts are homogeneous under the present oxidation conditions, they were regenerated and reused for up to two cycles.