Gallium phosphate, GaPO₄, is a piezoelectric crystal material homeotypic to quartz which features outstanding thermal stability up to 970°C, higher coupling than quartz and good temperature compensation for resonant applications. We report results in the research and development for high- temperature micro balance including thermo gravimetric analysis and for affinity sensor applications in aqueous liquids, such as biosensors. A sensor head for high-temperature micro balance applications with an integrated heater was developed. The clamping and heating device for the GaPO₄ resonator was designed for mechanical stability, reliable contacting and homogeneous temperature distribution. Cycles up to 600°C allow separation of volatile and fixed parts of layers deposited on the resonator surface and pyrolytic cleaning from carbonaceous residues. Another interesting field is the application of GaPO₄ for affinity measurements in liquids, e.g. for biosensors. An optimized crystal cut was developed and tested which allows temperature compensation in contact with an aqueous solution. A flow cell with a design for better reproducibility was constructed.