Nanocrystalline gas sensitive materials based on tin dioxide modified by Pd or Ru were synthesized via wet chemical route. The interaction of modified materials with CO and ammonia was studied by in situ DC-conductivity measurements and ex situ EPR spectroscopy. Modification by Pd yields the material highly sensitive to CO in low temperature region, while Ru-modified SnO2 is very sensitive to NH3 at raised temperature. We have detected that O-2(-) and OH center dot radicals are the main spin centers in unmodified nanocrystalline tin dioxide. The modification of tin dioxide by Pd and Ru is accompanied by formation of new spin centers in the samples: Pd+3 and Ru+3. The concentration of these paramagnetic species on the materials interacting with CO and ammonia gases decreased because of their transition to the diamagnetic state Pd+2, Pd-0 and Ru+4, respectively. (C) 2011 Elsevier B.V. All rights reserved.