Piezoelectric crystal microbalance devices based on gallium orthophosphate (GaPO4) have recently become commercially available. This material allows for operation at over 900 degrees C and therefore has potential as an analytical technique for the study of surface reactions at high temperatures. This paper describes preliminary work to assess the suitability of this technology for such applications. Change in oscillation frequency associated with temperature and gaseous environment is studied, and the ability to detect coke formation on a Ni-modified crystal is demonstrated. These results suggest that the technology can be developed as a low cost, high sensitivity gravimetric sensor for monitoring surface processes in high temperature chemical reactors such as reformers and solid oxide fuel cells (SOFCs)