The effect of γ-irradiation on 9 lanthanide acetylacetonate complexes was investigated. The degradation processes as a result of γ-irradiation were followed by infrared and ultraviolet spectrometry. It was found that the rate of degradation of the different complexes in methanol solutions are always lower than in solid state. The infrared spectra of the irradiated complexes indicated that the M—O bonds are the main degraded part of the complex and the rate of decrease in the intensities of these bands depends on the type of the coordinated lanthanide. In methanol solution, the rate of degradation and the G-values with the observed does were calculated and discussed in the light of coordination ability of the lanthanides as well as the degradation process.