The ortho, meta and para mono substituted anilines R-C6H4NH2 (R = 4-Me, 3-Me, 2-Me, 4-Et, 2-Et, 4-Br, 3-Br 2-Br; 4-Cl, 3-Cl, 2-Cl,4-F, 4-Pr-i, 4-Bu-t) were catalytically oxidised by H2O2, in the presence of cis-Mo(O)(2)(acac)(2), (acacH = CH3C(O)CH2C(O)CH3), producing the corresponding C-nitroso derivatives, R-C6H4NO. High conversions and selectivities were obtained.