A multi-storey gas sensor is constructed by stacking chemoresistor type gas sensing elements (CH1,CH2) and providing holes through each sensing element, at least in the region where the sensing layer (52) is formed, so that gas can pass from one sensing element to the next, through the sensing layers (52). A rich set of measurements can be obtained, enabling the characterisation/analysis of a gas under test with increased accuracy and/or the tailoring of the measurements to the particular gas-sensing application: notably by selecting appropriate combinations of materials for the sensing layers of the different gas-sensing elements and by varying the operating conditions applicable to the different gas-sensing elements notably by: taking measurements at respective times when different combinations of sensing layers in the stack are activated, at times when given sensing layers are heated to different temperatures or according to different heating profiles, and/or when selected sensing layers are exposed to UV light. Sensor sensitivity and selectivity can be increased by applying UV pulses of controlled duration. For miniaturization, and increased measurement-speed, each sensing element (CH1,CH2) may have a micro-hotplate architecture.