Unsaturated fatty acids and their derivatives were oxidized with aqueous hydrogen peroxide in two-phase systems in the presence of homogeneous catalysts Q(3){PO4[WO(O-2)(2)](4)}, where Q is [Bu (4) (n) ](3) (+), [C5H5N(n-C16H33)](3) (+) or [Me(n-C8H17)(3)N](3) (+). The reactions occurred under mild conditions (T < 100 A degrees C, atmospheric pressure), without organic solvents, and exhibited high conversion and selectivity.