The degradation of metobromuron (MB), isoproturon (IP), chlortoluron (CT), and chlorbromuron (CB) in heterogeneous photocatalytic solution via TiO2 was studied in detail. The influence of parameters such as TiO2 and herbicide concentration has been investigated and the optimal conditions for the abatement of the herbicides were found. The primary degradation of the herbicides was measured by HPLC analysis. A systematic study of their photodegradation has been made to assess structure–photoreactivity relationships. Correlation analysis showed that photoreactivity is associated with the molecular dipolar moment dependent on the differences in electronegativity of the substituents in the aromatic ring of each herbicide. The effect of the molecular charge distribution encoded in the dipolar moment was confirmed calculating electrostatic potential contour maps. It was also found that photoreactivity is inverse to the adsorption capability of these compounds on TiO2. Indeed, as confirmed in this paper, the extent of adsorption is not necessarily decisive in the evolution of the photochemical process and the photocatalytic reactions can occur independently of the degree of adsorption of the herbicides on TiO2.