The design of semiconductor-based photonic devices requires precise knowledge of the refractive index of the optical materials, a not constant parameter over the operating temperature range. However, the variation of the refractive index with the temperature, the thermo-optic coefficient, is itself temperature-dependent. A precise characterization of the thermo-optic coefficient in a wide temperature range is therefore essential for the design of nonlinear optical devices, active and passive integrated photonic devices and, more in general, for the semiconductor technology explored at different wavelengths, from the visible domain to the infrared or ultraviolet spectrum. In this paper, after an accurate ellipsometric and micro-Raman spectroscopy characterization, the temperature dependence of the thermo-optic coefficient (\documentclass[12pt]{minimal}
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				\begin{document}$$\frac{\partial n}{\partial T}$$\end{document}∂n∂T) for 4H-SiC and GaN in a wide range of temperature between room temperature to T = 500 K in the visible range spectrum, at a wavelength of λ = 632.8 nm, is experimentally evaluated. For this purpose, using the samples as a Fabry–Perot cavity, an interferometric technique is employed. The experimental results, for both semiconductors, show a linear dependence with a high determination coefficient, R2of 0.9648 and 0.958, for 4H-SiC and GaN, respectively, in the considered temperature range.