Ultrathin film deposition schemes often result in the growth of disordered structures whose surface chemistry is complex and can make predicting its reactivity difficult. Here, the surface reactivity of a disordered TiCN film is described using density functional theory calculations with simple cluster models of a general formula Ti4CxN4-x (0 < x < 4). These models allow for the simulation of adsorption on a Ti atom or along Ti-C and Ti-N surface sites. For the TiCN material, localized surface-adsorbate interactions are accurately predicted with the Ti 4CxN4-x models and compared with experiments. The energetics of adsorption on TiCN was simulated on our cluster models, using small probe molecules, and compared with the results of temperature programmed desorption studies. The robustness of the cluster model is validated by varying its atomic composition and utilizing different cluster terminations. This convenient approach can be used to design models of other disordered surfaces if localized interactions play the dominant role. © 2011 American Chemical Society.