Diffuse reflectance IR spectroscopy along with H₂ and CO adsorption measurements were applied to the study of 0.3% Pt/--Al₂O₃ catalyst precalcined in an air flow at 1023 K and then reduced at 773 and 973 K in hydrogen. Reduction of the catalyst at 973 K results in formation of supported metallic Pt particles in SMSI (strong metal-support interaction) state, as evidenced by the sharp decrease of Pt ability to chemisorb H, at room temperature as well as the significant lowering of thermal stability of linear Pt-CO complexes. Spectroscopically the effect of SMSI is manifested by the slight increase of the singleton frequency of linearly adsorbed CO from 2050 to 2060 cm^–1 together with the decrease of the dipole-dipole shift from 40 to 15 cm^–1 and by a large decrease of absorption band extinction.