Model calculations are performed with the semiempirical MO method SINDO1 to study the photocatalytic primary reactions at anatase particles in aqueous solution. The photochemical formation and migration of hydroxyl radicals is investigated at the particle surface, leading to the formation of hydrogen peroxide dr peroxide groups. These reactions represent the first step of the oxidative part of the photocatalytic decomposition of water into oxygen and hydrogen. The anatase particles are simulated by (TiO2)(n)(H2O)(m) clusters with structures corresponding to the anatase solid-state structure. With these clusters the photophysical properties of the anatase particles and the photoreactions are calculated on the SCF and CI level. The photoreactions are described by potential curves along selected reaction coordinates.