The reactivity of nanosized Ru(Pd,Pt)-doped SnO2, obtained by sol-gel synthesis, towards NO/Ar was investigated by Electron Paramagnetic Resonance (EPR), Mössbauer and Electrical Resistance measurements. A sensing mechanism was proposed that involves (i) the formation of bielectronic oxygen vacancies VO, (ii) their single-ionisation to VO
•, which injects electrons to SnO2 conduction band, (iii) the transfer of VO
• electrons to the transition metal centers reducing them to lower oxidation states. It was suggested that the electronic exchange between oxide and transition metal is responsible for the enhancement of the reactivity in doped SnO2 with respect to the undoped material.