Adsorption and photocatalytic oxidation of acetaldehyde have been investigated on TiO 2 and sulphate-modified TiO 2 films (denoted SO 4 -TiO 2 ). In situ Fourier transform infrared spectroscopy was used to study surface reactions as a function of time and number of experimental cycles. Spectral analysis and micro-kinetic modelling show that crotonaldehyde formation occurs spontaneously on TiO 2 but is impeded on SO 4 -TiO 2 , where instead acetaldehyde desorption is significant. Photo-oxidation yields significant amounts of formate on TiO 2 , and was identified as the rate-determining step and associated with site blocking.Significantly smaller amounts of formate were observed on SO 4 -TiO 2 , which is due to the acidity of this surface resulting in weaker bonding of aldehyde and carboxylate intermediate species. Our results are of considerable interest for applications to photocatalytic air purification and to surfaces with controlled wettability.