The reciprocity theory of McCumber [Phys. Rev. 136, A954 (1964)], which relates optical absorption and emission spectra between two energy level manifolds, was tested at temperatures down to 130 K on the F 32 4 → I 92 4 transition in Nd-doped glass. Deviations between calculated and measured spectra were observed for temperatures below 230 K in Nd:silicate and below 170 K in Nd:fluorozirconate glass. Below these temperatures, the calculated absorption is skewed to shorter wavelength and the calculated emission is skewed to longer wavelength. The magnitude of the deviation depends on the ratio of inhomogeneous linewidth to thermal energy kB T.