Low temperature oxidation of CO over alloy type Sn-Pt/SiO2 catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn-Pt/SiO2 catalysts. FTIR and Mossbauer spectroscopy has been used to follow compositional and structural changes of Sn-Pt/SiO2 catalysts during the catalytic run. The results show that the in situ formed, highly mobile "Snn+-Pt" ensemble sites are responsible for high activity, while formation of relatively stable SnOx type surface species are involved in the catalyst deactivation.