Among various single-metal oxides, In 2 O 3 , SnO 2 and ZnO were found to exhibit rather high sensitivities to CO at 300°C and above. Especially In 2 O 3 appeared to be most attractive in view of the selectivity to CO over H 2 . It was further found that modifications of In 2 O 3 with the addition of alkali metal carbonates, especially Rb 2 CO 3 , were very effective for enhancing the sensitivity and the selectivity to CO. The sensor element using Rb-In 2 O 3 could detect 500-4000 ppm CO in wet air at 300°C satisfactorily, while its cross-sensitivities to other gases such as H 2 , CH 4 , C 3 H 6 , and NO were comparatively small. Catalytic activity test and TPD measurements showed that, as a reason for the promoting effects, the addition of Rb 2 CO 3 to In 2 O 3 enhances the catalytic activity of CO oxidation. In addition, XPS data indicated that, as another reason, Rb 2 CO 3 forms the strongest p-n junctions with In 2 O 3 among alkali metal carbonates.