The three-phase hydrogenation of nitrobenzene catalysed by nanosized gold over titania was investigated in a slurry. Simultaneous in situ ATR-FTIR monitoring of the liquid phase and at the solid/liquid catalyst interface identified the species adsorbed on the catalyst and those in the liquid phase during the reaction. Nitrosobenzene was not detected analytically while the spectroscopic measurements strongly indicated phenylhydroxylamine as an intermediate reacting before desorbing from the catalyst surface. Under the same reaction conditions, azobenzene and hydrazobenzene were identified as intermediates during the hydrogenation of azoxybenzene to aniline. When nitrosobenzene or phenylhydroxylamine was alternately fed as reactant, azoxybenzene was produced via a disproportionation route. With the former, azoxybenzene was not further reduced by hydrogen because nitrosobenzene deactivated the catalyst. Combined with H₂ uptake, the spectroscopic measurements provided new insights into the reaction mechanism of the gold catalysed hydrogenation of nitrobenzene and an update of the corresponding kinetics.