A process for the preparation of 5-nitro-1,4-naphthoquinones of formula ##STR1## wherein R is hydrogen, a C.sub.1 -C.sub.4 alkyl radical or a C.sub.1 -C.sub.4 alkoxy radical, by oxidation of 1-nitronaphthalene or a suitably substituted 1-nitronaphthalene, which process comprises carrying out the reaction with manganese(III) sulfate as oxidizing agent or a mixture of manganese(III) sulfate and cerium sulfate.The maganese(III) sulfate consumed can be regenerated electrolytically with good current yield. The process is environmentally safe, as the oxidizing agent and the solvent, if any, can be reused.