Photochromism in V2O5 evaporated thin films has been investigated, especially with regard to its enhancement by adsorption of dimethylformamide (DMFA) molecules on the oxide surface. Measurements of the film optical properties in a wide energy range have been carried out giving a possibility to propose a model of the mechanism of the photochromic effect in the V2O5 films. At first, hydrogen atoms are detached from the adsorbed molecules of DMFA under light irradiation. They are predominantly localized near terminal oxygen atoms forming hydrogen bonds between hydrogen atoms and unshared electron pairs of the terminal oxygen atoms. Afterwards, under the action of light, some of these hydrogen atoms react chemically with oxygen atoms giving rise to the formation of oxygen vacancies. The 'blue' shift of the fundamental absorption edge is caused by two factors: (1) localization of hydrogen atoms near terminal oxygen atoms and (2) formation of oxygen vacancies due to breaking of V equals O bonds.