Abstract: Adsorption of carbon monoxide on a TiO ₂ (anatase) surface at room temperature has been studied by IR spectroscopy and volumetry. The obtained experimental data indicate weak adsorption of CO molecules on an exponentially inhomogeneous surface. It is established that adsorption enthalpy decreases according to the logarithmic law with a coverage increase in the range of 0.002–0.03. Values of the extinction coefficient of adsorbed CO for a heterogeneous CO/TiO ₂ system are calculated using the Bouguer–Lambert–Beer law and taking into account the Lorentz field correction. The validity conditions of the Bouguer–Lambert–Beer law for studying heterogeneous systems are described.