CuO-ZnO composites were fabricated by heating with infiltrating a cupric solution into a porous ZnO matrix. The composites possess a nonlinear, rectifying current-voltage character due to the presence of a p-n junction produced by the CuO and ZnO semiconductors. This junction is essential for the creation of voltage-dependent sensing properties of humidity and flammable gases. The forward current (CuO : positive bias) greatly increased with increasing the relative humidity, while the reverse current only slightly increased with an equivalent increase in the relative humidity. This asymmetric current change with the humidity is similar to that observed for conventional CuO and ZnO sintered specimens heterocontact produced by mechanically pressing the specimens together. The current was increased by the introduction of CO and H-2 (4000 p.p.m.) at 250 degrees C, with the current increase due to CO exceeding that of the H-2 in the measured bias region within +/-6 V. The utility of the new processing method for forming p-n semiconductor junctions open to the atmosphere has been shown.