Isotopic effects arising in the photolysis of benzophenones-H10/D10 mixtures in cyclohexane-D12 at room temperature were studied by CIDNP method. Nonlinear dependences of these effects on a relative concentration of benzophenone-D and benzophenone-H molecules have been obtained. To interpret the results, a theoretical model involving intermolecular and intramolecular energy transfers and radical-pair mechanism of CIDNP formation has been proposed. Nonlinearity of isotopic effects is determined by the difference in electron excitation energy between benzophenone-D and benzophenone-H molecules.