Isomerization reactions were found to be induced in dendrimers by weak infrared radiation. The rate constant was nonlinear to the intensity of the radiation, despite its very low intensity. We propose a possible mechanism for it. The radiation increases the average number of vibrational quanta, pushing up the effective temperature of the vibrational mode. The nonlinearity is brought about by the increase in the effective temperature at the reaction site as a result of vibrational transfer in the dendrimer. However, it is necessary for the vibrational decay to be very slow for this mechanism.