In this paper we discuss the effect of chlorine, added either as a part of the platinum precursor salt or after the noble metal impregnation, on the catalytic activity of Pt/Al2O3 for complete oxidation of toluene. Three different chlorinated and nonchlorinated platinum precursor salts have been used for the preparation of the catalysts. Characterisation of the catalysts has been carried out by N-2 adsorption, NH3 TPD, TPR, HREM, and CO adsorption under oxidising conditions followed by DRIFTS. The catalytic activity has been studied by toluene ignition curves and additional stability tests have been carried out under isothermal conditions. The inhibitory effect of chlorine has been proved and assigned to the formation of oxychlorided platinum species. A model of chlorine mobility, responsible for the observed stability trends, is also proposed.