Abstract: Thermo-optic and reactive mechanisms for label-free sensing of bio-particles are compared theoretically for Whispering Gallery Mode (WGM) resonators (sphere, toroid) formed from silica and stimulated into a first order equatorial mode. Although it has been expected that a thermooptic mechanism should “greatly enhance” wavelength shift signals accompanying protein binding on a silica WGM cavity having high Q(10⁸), for a combination of wavelength (680 nm), drive power (1 mW), and cavity size (43 µm radius), our calculations find no such enhancement. The possible reasons for this disparity are discussed