We have explored surface processes on Ti0.5Al0.5N(001) interacting with residual and environmental gases, namely O2, H2O and CO2, using ab initio molecular dynamics. Dissociative adsorption of O2 occurs on Ti sites, which are unusual sites, as Al2O3 is more stable than TiO2. This may be understood based on the electronic structure. We suggest that an increased Ti–O bond strength relative to Al–O surface bond strength is the electronic origin for the early stages of TiO2 formation on Ti0.5Al0.5N(001). Another unexpected atomic mechanism, identified as O covers the surface: Ti escapes from the Ti0.5Al0.5N(001)/O interface layer, generating vacancies, and hence enabling mobility at the interface. In the case of H2O and CO2, the dominating physical mechanism is dissociative adsorption, where O–H and N–H as well as C–O and Ti–O dipoles are formed, respectively. These fundamental surface processes are relevant for initial stages of oxidation, surface diffusion and nucleation of reaction layers upon exposure to residual and environmental gases.