In situ high temperature single crystal X-ray diffraction (XRD) experiments have been performed on a chemically quasi-ideal omphacite sample [(Ca0.49Na0.51)∑1(Mg0.46Al0.48Fe0.063+)∑1(Si1.97Al0.03)∑2O6], up to 1000°C. The lattice parameters were studied as a function of temperature, and their thermal expansion coefficients determined. The b and c cell edges show discontinuities as a function of temperature which are interpreted here in terms of intracrystalline cation diffusion processes. Structure refinements have been carried out using data collected at room temperature, at 800°C and at ambient conditions after cooling. The structural behaviour as a function of temperature of chemically quasi-ideal omphacites is compared with those of jadeite and diopside.