Joint i.r. and microcalorimetric work has shown that two types of CO adsorb on pure anatase: one (species A) absorbing at 2184–2188 cm^–1, has an adsorption heat of ca. 52 kJ mol^–1, the other (species B), absorbing at 2203–2208 cm^–1, has an adsorption heat of 69 kJ mol^–1. When sulphates are present at the surface, left from the process of preparation, species B is suppressed, and species A absorbs at 2190–2196 cm^–1, with an adsorption heat of 59 kJ mol^–1. The CO species have been characterized also through their molar extinction coefficient and entropy of adsorption at vanishing coverage. By comparison with other data on similar systems it can be shown that, when adsorbing CO on non-d cations, these physical properties are related to one another in simple ways. In particular, it was found that the adsorption standard entropy shows a compensation effect with adsorption enthalpy. The adsorption of species B is more or less Langmuirian, whereas the adsorption on sites A is non-ideal, as shown by several features (i.e. non-Langmuirian isotherms, shifts of the CO stretching frequency, changes on differential heats), probably because inductive effects are readily transmitted through the semiconductive TiO₂ solid.