Interaction of various reducing and oxidizing gases with ultrathin SnO₂ film at room temperature has been studied to investigate the selectivity of SnO₂ film towards ammonia. SnO₂ thin films show no response with O₂, H₂, CO, ethanol and methane. In presence of H₂S and SO₂ gases conductivity of the film increases where as film conductivity decreases with ammonia and NO₂. With ammonia gas, the response is fast and >90% recovery takes place within 30 min where as with NO_2-recovery is partial. The impedance study shows distinctly different frequency dependent responses towards NH₃ and NO₂ indicating specificity to ammonia in presence of NO₂.