The solid-state thermal behaviour of gamma-Al2O3 doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations : gamma-Al2O3 --> delta-Al2O3 --> theta-Al2O3 --> alpha-Al2O3 was observed for Cr2O3-doped alumina samples between 500 degrees C and 1100 degrees C. Rapid grain growth occurred at temperatures close to 1100 degrees C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 degrees C corresponded to the resonance of beta-Cr3+ in an amorphous Cr oxide impregnated onto the c-and alumina support. The change of ESR spectrum indicated the existence of Cr3+, suggesting the formation of a solid solution with the same structures as delta-Al2O3 and/or theta-Al2O3 at 800-1000 degrees C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000h(-1). The modified Al2O3 catalyst heat-treated at 1000 degrees C in air showed removal conversion of 100% for hydrocarbon (C3H6), 92% for CO and 5% for NO at 550 degrees C. Present results suggest that Cr-modification to Al2O3 leads to catalytic improvement with good thermal durability.