We report the synthesis of SnO2?Cu2O n?p core?shell nanowires (C?S NWs) and their use as chemiresistive sensors for detecting trace amounts of gas. The n?p C?S NWs were synthesized by a two-step process, in which the core SnO2 nanowires were prepared by the vapor growth technique and subsequently the Cu2O shell layers were deposited by atomic layer deposition. A systematic investigation of the sensing capabilities of the n?p C?S NWs, particularly as a function of shell thickness, revealed the underlying sensing mechanism. The radial modulation of the hole-accumulation layer is intensified under shells thinner than the Debye length. On the other hand, the contribution of volume fraction to resistance modulation is weakened. By the combination of these two effects, an optimal sensing performance for reducing gases is obtained for a critical p-shell thickness. In contrast, the formation of p-shell layers deteriorates the NO2-sensing performance by blocking the expansion of the hole-accumulation layer due to the presence of p?n heterointerface.