Uniform SnO2 nanorod arrays have been deposited at low temperature by plasma-enhanced chemical vapor deposition (PECVD). ZnO surface modification is used to improve the selectivity of the SnO2 nanorod sensor to H2 gas. The ZnO-modified SnO2 nanorod sensor shows a normal n-type response to 100 ppm CO, NH3, and CH4 reducing gas whereas it exhibits concentration-dependent n–p–n transitions for its sensing response to H2 gas. This abnormal sensing behavior can be explained by the formation of n-ZnO/p-Zn-O-Sn/n-SnO2 heterojunction structures. The gas sensors can be used in highly selective H2 sensing and this study also opens up a general approach for tailoring the selectivity of gas sensors by surface modification.