The thermal equation of state of a natural kyanite has been investigated with a DIA-type, cubic-anvil apparatus (SAM85) combined with an energy-dispersive synchrotron X-ray radiation technique up to 8.55 GPa and 1273 K. No phase transition was observed in the studied pressuretemperature (P-T ) range. The Le Bail full profile refinement technique was used to derive the unit-cell parameters. By fixing the bulk modulus K₀ as 196 GPa and its pressure derivative K^'₀ as 4, our P-V (volume)-T data were fitted to the high temperature BircheMurnaghan equation of state. The obtained parameters for the kyanite are: V₀=294.05(9)(10^-10)³, α₀2.53(11)× 10^-5K^-1 and (?K/?T )P =-0.021(8) GPa?K^-1. These parameters have been combined with other experimentally-measured thermodynamic data for the relevant phases to calculate the P-T locus of the reaction kyanite = stishovite+ corundum. With this thermodynamically constrained phase boundary, previous high-pressure phase equilibrium experimental studies with the multi-anvil press have been evaluated.