We report on the synthesis of MPO4:Ce3+ (M=Y,Gd) phosphor particles via a liquid-fed flame aerosol route with the objective of producing new luminescent sensor particles for aerosol-based thermometry in high-temperature gas flows. The luminescence, structural and morphological properties of the particles are characterised with respect to the host metal, dopant concentration, and the aerosol synthesis temperature. The optimal materials are found to be YPO4:Ce3+ 5 mol% particles synthesised at a temperature of 1500 K which have several essential features for flow thermometry: the particles are ~1 µm in diameter; possess a thermally-stable crystal structure; emit bright luminescence (4×1013 photons per mg of particles at 300 K) with a suitably short lifetime (~20 ns at 300 K); and have a T10 thermal quenching temperature of nearly 1300 K. Luminescence data are used to determine the performance of the particles as tracers for two-colour temperature imaging in gas flows. A single shot temperature precision of better than 10% at 1200 K can be achieved, representing a significant increase of over 200 K in the high temperature limit of gas thermometry.