The temperature sensitivity of the chemical shift of water (approximately 0.01 ppm/°C) provides a potential method to monitor temperature changesinvivoorinvitrothrough the changes in phase of a gradient‐echo magnetic resonance (MR) image. This relation was studied at 1.5 T in gel materials andinvivoin canine brain and muscle tissue, heated with a radio frequency (rf) annular phased array hyperthermia antenna. The rf fields associated with heating (130 MHz) and imaging (64 MHz) were decoupled using bandpass filters providing isolation in excess of 100 dB, thus allowing simultaneous imaging and rf heating without deterioration of the MR image signal‐to‐noise ratio. In a gel, temperature sensitivity of the MR image phase was observed to be (4.41±0.02) phase degrees/°C forTe=20 ms, which allowed temperature changes of 0.22 °C to be resolved for a 50‐mm3region in less than 10 s of data acquisition.Invivo, forTe=20 ms, the temperature sensitivity was (3.2±0.1) phase degrees/°C for brain tissue, (3.1±0.1) phase degrees/°C for muscle, and (3.0±0.2) phase degrees/°C for a muscle tumor (sarcoma), allowing temperature changes of 0.6 °C to be resolved in a 16‐mm3volume in less than 10 s of data acquisition. We conclude that, while the technique is very sensitive to magnetic field inhomogeneity, stability, and subject motion, it appears to be useful forinvivotemperature change measurement.