The ability to make localized temperature measurements in tissue during hyperthermia treatment of cancer is an essential factor in optimizing its efficacy. To this end we have developed and evaluated the complex tris(ethylenediamine) cobalt(III) trichloride as a temperature sensor by determining the temperature dependence of it 59Co nuclear magnetic resonance chemical shift. Encapsulating this complex within liposomes targets the agent to the reticuloendothelial system. Temperature changes of the order of 0.1 degrees C have been measured in vivo on rats, and the half-life of the complex within the body determined by plasma emission spectroscopy.