We have measured the first and second derivatives of the refractive index with respect to temperature along the z- and y-axes for two nonlinear optical materials, RbTiOPC4 and RbTiOAsO4. The measurements were done using an interferometric setup in the wavelength range of 532-1580 nm and in the temperature range of 25-200°C. In addition, the thermal expansion coefficients were measured in the x direction. Experimental results of nonlinear frequency conversion in RbTiOAsO4 were used to explore the temperature dependence of the refractive index in the z-axis near 3.5 microns. We have derived thermal-dependent dispersion equations for the two materials. Calculations based on these dispersion equations are in good agreement with previously reported measurements. The thermal dependence of the refractive index of RbTiOAsO4 is greater than that of RbTiOPO4 by approximately 15% and 70% in the z- and y-axes, respectively. Hence RbTiOPO 4 is more appropriate for applications that require immunity to thermal lensing, whereas RbTiOAsO4 is suitable for realizing temperature-tuned nonlinear devices.