Recent advances in the development of several types of sensors for monitoring chemistry parameters in aqueous solutions at high temperatures are reviewed. New types of membrane and metal/metal oxide combination pH sensors have been developed and tested in high temperature solutions, including supercritical aqueous systems. Among these is a glass pH sensor that has been employed at temperatures as high as 250°C. Sensors for the continuous monitoring of conductivity, electrochemical noise, and dissolved hydrogen and oxygen are under development. The definition and standardization of pH at supercritical temperatures is discussed. Preliminary results indicate the importance of further developing viable experimental techniques for defining the chemistry of supercritical aqueous environments.