Formation of paramagnetic centers arising in highly disordered MoO3 films due to photoinjection of hydrogen at room and liquid helium temperatures has been investigated. At room temperature the concentration of Mo5+, paramagnetic centers very quickly comes to a steady-state value, whereas at liquid helium temperature it grows continuously with exposure during the whole illumination time. Formation of free radicals forming due to the detachment of hydrogen atoms from the hydrogen-donor molecules has been observed at liquid helium temperature. Its reaction rate decreases twice during the illumination, whereas the formation rate of the Mo5+ centers decreases more than an order of the magnitude. This difference in behavior has been attributed to the pairing of the Mo5+ centers. (C) 2008 Elsevier B.V. All rights reserved.