Solar photodegradation in aqueous solutions of phenol, 4-chlorophenol, 2,4-dichlorophenol, bromoxynil, atrazine, imidachloprid and oxamyl, in the presence of two photocatalysts titanium dioxide TiO2 and sodium decatungstate Na4W10O32 is reported. TiO2 appears to be the most efficient photocatalyst concerning the rate of photodegradation and mineralization of the pure compounds. However, when the pesticides are used in formulated solutions, the decatungstate anion becomes as efficient or even more efficient than TiO2. The mechanism of the photodegradation in the presence of Na4W10O32 is investigated by nanosecond flash-photolysis experiments, and is compared to the one proposed for TiO2. The difference of reactivity between the two photocatalysts is accounted by the different primary processes of the mechanisms. The formulation effect is investigated with two model surfactants SDS and Triton X100, and 2-chlorophenol as a model pesticide.