We carried out the experiments which show unambiguously that not one, as it was commonly accepted, but three optical absorption bands corresponding to different color centers arise in the nanosized MoO3 films due to hydrogen atoms. Varying the conditions for the photoinjection of hydrogen into the MoO3 films, it is possible to change drastically the correlation between the concentrations of different arising centers, which in turn yields the great difference in the optical absorption spectra. Our findings are crucial for understanding of the nature of electrochromism and photochromism not only in MoO3 films but also in the series of the transition metal oxide films. (C) 2011 Elsevier B.V. All rights reserved.