We investigate Nb5+ doped alkaline earth ferrates (SrFe1-xNbxO3 and BaFe1-xNbxO3, where x = 0.0-0.5), as toxic gas sensors with emphasis on their role in NO, sensing. We show that the sensitivity of these materials for NO, depends on the A type cation (Sr or Ba), and also that the concentration of Nb5+ is crucial. The largest response is observed for BaFe0.65Nb0.35O3 at 350 degrees C. We observe a p- to n-type transition around 500 degrees C, on exposure to the analyte gas. We find that the effects of other interfering gases (hydrogen, oxygen and water vapour) on the sensitivity of this material towards NO, are small. Extended X-ray absorption fine structure (EXAFS) spectroscopic techniques have been applied to gain information on the local structure in these materials. EXAFS results show that the local environment of the dopant is different from that of the host atoms. Doping with Nb5+ significantly alters the local environment of the Fe species. The effect is greater for BaFeO3 than for SrFeO3; in the former case a local structural change is observed around the Fe. In both cases it is found that the Fe-O bond lengths increase as we increase the concentration of the dopant ion. This increase in bond length can be attributed to the conversion of Fe(IV) into Fe(III). In contrast, the Nb-O bond lengths do not vary significantly on changing the Nb5+ concentration, indicating that the dopant environment is independent of its concentration.