The optical properties of tin oxide-coated aluminium and anodized aluminium are reported. It is shown that after annealing the coating on bare aluminium becomes highly absorbing while the coating on anodized aluminium is stable at 480 degrees C and does not change. Optical modelling and Auger electron spectroscopy are used to explore the possible reason behind this behaviour. It is suggested that the most feasible explanation is an interfacial reaction between the metal and the oxide. Optical constants for a mixture of aluminium and tin oxide calculated according to the Bruggeman effective medium theory are presented, together with calculated reflectance spectra that fit the experimental spectra. The results demonstrate the high sensitivity of optical spectroscopy for the analysis of coated surfaces.