Adsorption of molecules is a fundamental step in all heterogeneous catalytic reactions. Nevertheless, the basic mechanism by which photon-mediated adsorption processes occur on solid surfaces is poorly understood, mainly because they involve excited catalyst states that complicate the analysis. Here we demonstrate a method by which density functional theory (DFT) can be used to quantify photoinduced adsorption processes on transition metal oxides and reveal the fundamental nature of these reactions. Specifically, the photoadsorption of SO2 on TiO2(101) has been investigated by using a combination of DFT and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The combined experimental and theoretical approach gives a detailed description of the photocatalytic desulfurization process on TiO2, in which sulfate forms as a stable surface product that is known to poison the catalytic surface. This work identifies surface-SO42- as the sulfate species responsible for the surface poisoning and shows how this product can be obtained from a stepwise oxidation of SO2 on TiO2(101). Initially, the molecule binds to a lattice O2- ion through a photomediated adsorption process and forms surface sulfite, which is subsequently oxidized into surface-SO42- by transfer of a neutral oxygen from an adsorbed O2 molecule. The work further explains how the infrared spectra associated with this oxidation product change during interactions with water and surface hydroxyl groups, which can be used as fingerprints for the surface reactions. The approach outlined here can be generalized to other photo- and electrocatalytic transition metal oxide systems.