The reactions of benzaldehyde at platinum in 0.1 M HClO4 were studied by means of differential electrochemical mass spectrometry (DEMS) and in situ Fourier transform infrared (FTIR) spectroscopy combined with cyclic voltammetry. With the application of DEMS, it was established that this compound oxidizes completely to CO2 at E > 0.60 V (RHE) and reduces to toluene, benzene, and cyclohexane in the potential region for hydrogen adsorption and H-2 evolution. No partial or total hydrogenation of toluene was observed. In situ FTIR measurements show that the oxidation of benzaldehyde also produces adsorbed benzoate ions (band at 1376 cm(-1)) at about 0.60 V (RHE), which are protonated to benzoic acid (band at 1273 cm(-1)) upon desorption at E > 1.0 V (RHE). The assignment is confirmed by FTIR experiments with benzoic acid. Fragmentation of benzaldehyde during adsorption is confirmed by the presence of vibration bands at 2002 and 1807 cm(-1) assigned to lineal and bridge bonded CO, respectively, and at 1240 cm(-1) due to (COH)(ad). The results are compared with previous studies on the reactivity of benzyl alcohol, benzoic acid, toluene, and benzene.