The effects of irradiation with 1.17‐ and 1.33‐MeV60Co gamma rays, 0.662‐MeV137Cs gamma rays, and thermal neutrons on the carrier concentration inn‐type CdS and CdTe have been studied by means of Hall‐effect measurements. Irradiation of these materials at room temperature did not introduce any new energy levels in the range from 0.02 to 0.2 eV below the conduction band (the range accessible by experiment), although the density of levels initially present in this interval was altered and, under certain conditions, deep‐lying acceptor states were produced. The most significant observation was the change in the nature of the effect as the energy of the incident gamma rays was changed. For example,137Cs photons introduce acceptors in CdS and donors in CdTe, while60Co photons produce the inverse effect. Atomic displacement calculations indicate that the lower‐energy137Cs photons should preferentially displace sulfur atoms in CdS and cadmium atoms in CdTe; the opposite is calculated for60Co. Hence, these studies suggest that the net result of displacement of a chalcogenide atom is acceptor introduction, with donor introduction resulting from displacement of the cadmium atom. Thermal neutron irradiation of both materials introduces acceptors.