An analysis is made of the photodissociation and photoionization processes in the mesosphere due to the solar H Lyman-alpha line. The irradiance of the line and its variation with solar activity are considered in the determination of the photodissociation of CH4, CO2, H2O and O2, and of the photoionization of NO. Lyman-alpha contributes directly to these processes in the mesosphere after its absorption, which depends on wavelength and temperature, by molecular oxygen. The H Lyman-alpha radiation considered for mesospheric processes is characterized by a profile of an emission line with a central reversal, and wings extending to about ± 1.75 A where the intensity reaches about 1% of that of the peak. Simple formulae are deduced for the photodissociation optical depths and frequencies and these take into account the various solar activity conditions and the different spectral characteristics of each molecule.