Minute amounts of supported Au nanoparticles on TiO2 (0.1 mol%) catalyze quantitatively at room temperature the transfer hydrogenation of azoarenes to hydrazoarenes by ammonia borane complex, in ethanol as solvent. At 1 mol% catalyst loading level and 2.5 molar equivalents of ammonia borane the process occurs instantaneously. The catalyst is recyclable and reusable. In the absence of reducing agent, hydrazoarenes undergo smooth Au/TiO2-catalyzed aerobic oxidation back to azoarenes.