A new photocatalytic composition is described that is designed for percolation in road surfaces. The composition contains no aggregates, but consists of cement, a photocatalyst, a superfluidifier, a viscosity modifier and an antifoaming agent, in suitable proportions. The composition is applied to porous open-graded asphalt courses, completely filling the cavities down to a thickness coming between 30 and 50 mm. The treated asphalt acquires a strong, durable photocatalytic activity, even when it is submitted to heavy mechanical loads and adverse weather conditions, as in the case of road surfaces in and out of town, liable to intense traffic.