The mutual solubility in the system corundum-hematite [a-(Al, Fe 3þ ) 2 O 3 ] was investigated experimentally using both synthetic and natural materials. Mixtures of g-Al 2 O 3 and a-Fe 2 O 3 (weight ratios of 8:2 and 10:1) were used as starting materials for synthesis experiments in air at 800-1300 C with run times of 7-34 days. Experiments at 8-40 kbar and 490-1100 C were performed in a piston-cylinder apparatus (run times of 0Á8-7Á4 days) using a natural diasporite consisting of 60-70 vol. % diaspore and 20-30 vol. % Ti-hematite. During the diasporite-corundite transformation, the FeTiO 3 component (12-18 mol %) of Ti-hematite only slightly increased, implying that oxygen fugacity was maintained at high values. Run products were studied by electron microprobe and X-ray diffraction (Rietveld) techniques. An essentially linear volume of mixing exists in the solid solution with a slight positive deviation at the hematite side. Up to 1000 C, corundum contains <4 mol % Fe 2 O 3 and hematite <10 mol % Al 2 O 3 ; at 1200 C these amounts increase to 9Á3 and 17Á0 mol %, respectively. At 1300 C hematite was no longer stable and a-(Al 0Á88 Fe 0Á12 3þ O 3 ) coexists with the orthorhombic phase Fe 0Á53 3þ Al 0Á47 O 3 . The present results agree with corundum (solvus) compositions obtained in previous studies but indicate a larger solubility of Al in hematite. The miscibility gap in the solution can be modelled with an asymmetric Margules equation with interaction parameters (2s uncertainties): W H Cor ¼ 89 AE 10 kJ/mol; W S Cor ¼ 23 AE 8 J/(Kmol); W H Hem ¼ 91 AE 14 kJ/mol; W S Hem ¼ 14 AE 10 J/(Kmol). Application of the corundumhematite solution as a solvus geothermometer is limited because of the scarcity of suitable rock compositions.