An analysis of the photodissociation of water vapor has been made for mesospheric conditions. The simultaneous action of two spectral ranges, namely the Lyman-alpha radiation and the O2 Schumann-Runge bands, must be considered. The H2O photodissociation frequency owing to Lyman-alpha is represented by J(H2O,Ly-α) = J∞exp <-4.17¿10-19N0.917>, where N is the total number of O2 absorbing molecules, and J∞ = (3.5¿1.0)¿10-6 s-1 for quiet sun conditions and reaches (1.25¿0.35)¿10-5 s-1 for very active sun conditions. The H2O photodissociation frequency resulting from the O2 Schumann-Runge band spectral range is represented by J(H2O,SRB) = J∞(H2O, SRB) exp <-4.8¿10-10N0.466> for N?1020 cm-2, and J(H2O, SRB) = J∞(H2O, SRB) exp <-4.6¿10-8N0.366> for 1020<N?1022 cm-2. The mean value which has been adopted for J∞(H2O, SRB) = (1.3¿0.3)¿10-6 s-1 while a median value could be (1¿0.5)¿10-6s-1 based on all observational data.