CO 2 chemisorption on SrTiO 3 (001) surfaces is studied using ab initio calculations in order to establish new chemical sensing mechanisms. We find that CO 2 adsorption opens the material band gap, however, while the adsorption on the TiO 2 -terminated surface neutralizes surface states at the valence band (VB) maximum, CO 2 on the SrO-terminated surfaces suppresses the conduction band (CB) minimum. For the TiO 2 -terminated surface, the effect is explained by the passivation of dangling bonds, whereas for the SrO-terminated surface, the suppression is caused by the surface relaxation. Modulation of the VB states implies a more direct change in charge distribution, and thus the induced change in band gap is more prominent at the TiO 2 termination. Further, we show that both CO 2 adsorption energy and surface band gap are strongly dependent on CO 2 coverage, suggesting that the observed effect can be utilized for sensing application in a wide range of CO 2 concentrations.