1. Rhodium trichloride tetrahydrate without promoters catalyzes the synthesis of isocyanates by the carbonylation of azobenzenes. Pyridine, compounds of vanadium, molybdenum, and iron acetylacetonate inhibit this reaction. 2. Molybdic acid, its salts, molybdenum trioxide, vanadium pentoxide, the vanadium(IV) oxychloride complex with dioxane, organometallic compounds of vanadium, iron and vanadium monoxide acetylacetonates promote rhodium trichloride in carbonylating nitro compounds with carbon monoxide. 3. Nitro compounds are converted into isocyanates in the presence of rhodium catalysts, bypassing the stage of formation and carbonylation of azo compounds as a result of the direct carbonylation of the nitrene formed as an intermediate.