Tin oxide nanocrystals, both pure and Cu²⁺ and Fe³⁺ doped, have been prepared by a sol–gel process. The response of these materials to carbon monoxide in dry air has been investigated as a function of annealing temperature. The growth of the crystallites was monitored by XRPD from room temperature to about 900 °C and the response of the material to CO was studied for materials annealed over that temperature range. All of the materials were shown to respond to low concentrations of CO with a narrow peak in sensitivity at an operating temperature of about 200 °C. A good response to CO was also observed at an operating temperature of about 400 °C. No improvements in selectivity to CO were observed by the addition of either of the cation dopants. The sensitivity to CO was shown to decrease as crystallite size increased. The addition of the metal cation dopants impeded crystallite growth. Our previously reported Cu K-edge and Fe K-edge EXAFS measurements, on the Cu²⁺ and Fe³⁺ doped materials respectively, showed the dopant cations to move from ordered Sn⁴⁺ substitutional lattice sites in the as-prepared materials to more disordered regions, most likely the surface regions, as the materials were annealed. This dopant migration begins at about 400 °C and is accompanied by a corresponding large decrease in response to CO at an operating temperature of about 200 °C (the peak in sensitivity). This is attributed to the migration of the dopants to the surface of the crystallites and speculative explanations are given. The reduction in response of the same materials at an operating temperature of about 400 °C is not so large, indicating the response mechanisms at 200 and 400 °C to be different.