The thermo-optic coefficient (dn/dT) of crystalline silicon has been critically analyzed in the temperature range 300–600 K, at the fiber optic communication wavelength of 1.5 μm. The temperature dependence has been attributed to the variation of the interband transition energies at some critical points of the silicon band structure. The experimental data have been fitted using single and double oscillator models. In particular, the double oscillator model, which is physically correlated to the silicon band structure, has been exploited to extrapolate the temperature dependence of the interband transition energies at some points (critical points) of the combined density of states. The extracted parameters are in good agreement with the data reported in the literature. Finally, in connection with both of the oscillator approximations, an analysis based on thermodynamic considerations is carried out, and electron–hole formation entropy and specific heat are calculated. The consistency of the obtained results validate the reliability of the proposed analysis. © 2000 American Institute of Physics.