This paper reports results on a TiO2 (anatase)-based CO sensor and proposes a possible surface-controlled sensing mechanism. The complex plane analysis of the ac electrical data provides quantitative evidence supporting the mechanism. The grain-boundary capacitance and conductance, extracted from the impedance plane, were observed to increase with the CO concentration. This effect is attributed to a change in the depletion region thickness and barrier height at the TiO2 intergranular contact. The analysis of the electrical data combined with x-ray diffraction and x-ray photoelectron spectroscopy observations support the proposed sensing mechanism involving CO adsorption and ionization on the titania surface, and not an oxidation-reduction type reaction as observed in most oxide-based sensors.