We report the measurement of variation in refractive index vs. temperature for organic-inorganic slab waveguides. The following parameters have been obtained: refractive index n; thermo-optic constant dn/dT, and linear thermal expansion coefficient. The refractive index has been measured between. approximately 10 and 80 degreesC. For both polarization TE (Transverse Electric) and TM (Transverse Magnetic), the slopes (dn/dT) are linear over the temperature range with a correlation coefficient r greater than or equal to 0.997 at a wavelength of 632.8 nm. Using the prism coupling method we verified that the refractive index variations follow the Lorentz-Lorenz law. A zero-birefringence is obtained. The thermo-optic coefficient for different slab waveguides was found to be almost constant and not exceeding 10(-4)/degreesC.