In Si and Ge, the optical dispersion parameters (single-oscillator energyEo, dispersion energyEdand bond energy gapEgdeveloped by Wemple and DiDomenico, and Phillips) have been analysed in the temperature range 100-300 K using data obtained by Icenogleet al. EoandEgexhibit a very small temperature dependence in both materials. The thermal coefficients of the dispersion energy, dEd/dT, have opposite signs (Si, –41·9 × 10−4eVK−1; Ge, +37·7 × 10−4eVK−1).