In the Bayer process, alumina trihydrate is precipitated from green liquor by seeding with fine recycled hydrate. The goal of the precipitation section is to maximise alumina yield while maintaining product quality (particle size and strength) and seed balance. Control variables in precipitation are temperature, starting alumina/caustic ratio, seed charge, caustic concentration and holding time. Improved control of the precipitation stage should produce less fines and stronger alumina which will in turn generate less fines in downstream calcining and handling. The optimisation of alumina precipitation would benefit from the development of suitable on-line particle size analysers to monitor the proportions of both coarse (plus about 75 microns) and fine (minus about 20 microns) alumina. At present, there are no commercially available on-line particle size monitors capable of this measurement. CSIRO is developing a new on-line particle size analyser to operate in the hot, caustic and scaling environment of an alumina precipitation plant. The analyser is based on a combination of ultrasonic and gamma-ray transmission measurements. Laboratory measurements have been performed on a range of well characterised particulate samples of glass and alumina in caustic solutions and synthetic Bayer liquors. For alumina samples at solids fraction from 10 to 30 wt per cent in synthetic Bayer liquors, the method simultaneously determined d50 to within 3.8 microns, caustic concentration to within 1.32 g/l and solids weight fraction to within 0.5 wt per cent.