Here we report the influence of annealing temperature on the gas sensing performance of ZnO/SnO2 heterojunction thin films fabricated using a low-cost sol-gel drop coating technique. The films were annealed at 400 °C,450 °C,500 °C,550 °C and their structural, morphological and sensing characteristics were systematically examined. X-ray diffraction and scanning electron microscopy analyses reveal that annealing at 550 °C leads to improved crystallinity, enhanced grain connectivity and the formation of a well-connected ZnO/SnO2 heterojunction structure. X-ray photoelectron spectroscopy confirms the presence of Zn2+, Sn4+ and surface oxygen species indicating the stable oxide formation and modified surface chemistry after high temperature annealing. Gas sensing measurements toward ethanol conducted over an operating temperature range of 25-200 °C revealed that the film annealed at 550 °C achieving a maximum steady state response of approximately 4.0 × 104% at 200 °C for 100 ppm ethanol. The enhanced sensing performance is due to the combined effects of improved microstructure, interfacial charge transport and thermally activated surface oxygen reactions. These results highlight the role of annealing in altering the structure which boosts the sensing behavior of ZnO/SnO2 heterojunctions and demonstrates the potential of this low-cost fabrication approach for high performance and humidity tolerant chemiresistive ethanol sensing applications.