Sn-Pt/SiO2 catalysts used in room temperature oxidation of CO were characterized by in situ techniques applying both Mossbauer and FTIR spectroscopy. The catalysts were prepared by an organometallic method (CSR) using Sn-119(CH3)(4). The Mossbauer spectra revealed the presence of platinum-rich [PtSn(a)] and tin-rich [PtSn(b)] alloy species as the main tin-containing components accounting for more than 85% of tin after activation of the catalysts in hydrogen at 573 K. The results of Mossbauer spectroscopy showed that the oxidation of the less stable tin-rich PtSn(b) component, with looser Sn-Pt bonds, is primarily involved in surface reactions taking place in the low temperature CO oxidation. The oxidation of both PtSn(b) and PtSn(a) species leads to the formation of new species and sites, such as (i) a tin oxide phase, (ii) a mobile Sn4+(sf) species, (iii) platinum sites, and (iv) the appearance of a third alloy species, PtSn (1 : 1). The net result of this transformation is the formation of highly active "Sn4+-Pt" ensemble sites, where Sn4+ sites are in the atomic closeness of Pt. In both CO oxidation and subsequent reactivation in hydrogen a reversible PtSn(b) <----> Sn4+ + Pt interconversion takes place at room temperature. The probability of the Mossbauer effect, f(A) (approximated by d ln(A(300)/A(77))/dT), indicates the surface location of the involved components. The results of in situ FTIR spectroscopy provided further proof for the above interconversion and unambiguous evidences of the involvement of both Pt-rich PtSn(a) and Sn-rich PtSn(b) alloy species in the above interconversion. The catalytic experiments are in full accordance with the results of spectroscopic measurements. The possible mode of activation of the CO molecule over the (110) surface of the PtSn (1 : 1) alloy was also modeled and calculated.