We present a theoretical and experimental study of the frequency response of the photothermal effect in silicon waveguides. The effect is studied for modulation frequencies up to 3 GHz using integrated photonic circuits in Mach-Zehnder and ring oscillator configurations. The thermal behavior of silicon waveguides is described by a diffusive substrate heating model. In the low-frequency regime, the frequency response follows a -log (f) dependence, while a f-1/2 dependence is found in the high-frequency regime. Close agreement between theory and experiment allows for the accurate extraction of the photothermal absorption coefficient.