A gas sensor comprising: a housing having an inlet and an outlet for gas to be analysed; two or more chemoresistor type gas-sensing elements (CH1,CH2) stacked in the housing to form a multi-storey structure comprising: a first storey comprising a sensing layer provided on an insulating layer, and measurement electrodes provided in contact with the sensing layer, and a second storey comprising a second sensing layer provided on an insulating layer, and second measurement electrodes provided in contact with the second sensing layer, said second sensing layer being a layer of selectively-activatable material, the first and second storeys being formed by adjacent gas-sensing elements (CH1,CH2) in the stack, and the first storey and the second storey each constituting a chemoresistor type gas sensing element; and an activation unit for controlling activation of the sensing layer of the second storey; wherein the housing and the stack of chemoresistor type gas-sensing elements (CH1,CH2) define, for gas undergoing analysis, a gas flow path passing through the sensing layer of the second storey before reaching the sensing layer of the first storey whereby, in use, the composition of gas reaching the first storey is influenced by the presence and operation of the sensing layer of the second storey; and wherein there is further provided control means adapted to take measurements via the measurement electrodes of the first storey at times when the second sensing layer of the second storey is activated and de-activated, respectively, and to process said measurements, as well as an electrical signal from measurement electrodes of the second storey, to discriminate gases presented to the gas sensor. a housing having an inlet and an outlet for gas to be analysed; two or more chemoresistor type gas-sensing elements (CH1,CH2) stacked in the housing to form a multi-storey structure comprising: a first storey comprising a sensing layer provided on an insulating layer, and measurement electrodes provided in contact with the sensing layer, and a second storey comprising a second sensing layer provided on an insulating layer, and second measurement electrodes provided in contact with the second sensing layer, said second sensing layer being a layer of selectively-activatable material, the first and second storeys being formed by adjacent gas-sensing elements (CH1,CH2) in the stack, and the first storey and the second storey each constituting a chemoresistor type gas sensing element; and an activation unit for controlling activation of the sensing layer of the second storey; wherein the housing and the stack of chemoresistor type gas-sensing elements (CH1,CH2) define, for gas undergoing analysis, a gas flow path passing through the sensing layer of the second storey before reaching the sensing layer of the first storey whereby, in use, the composition of gas reaching the first storey is influenced by the presence and operation of the sensing layer of the second storey; and wherein there is further provided control means adapted to take measurements via the measurement electrodes of the first storey at times when the second sensing layer of the second storey is activated and de-activated, respectively, and to process said measurements, as well as an electrical signal from measurement electrodes of the second storey, to discriminate gases presented to the gas sensor. a first storey comprising a sensing layer provided on an insulating layer, and measurement electrodes provided in contact with the sensing layer, and a second storey comprising a second sensing layer provided on an insulating layer, and second measurement electrodes provided in contact with the second sensing layer, said second sensing layer being a layer of selectively-activatable material, the first and second storeys being formed by adjacent gas-sensing elements (CH1,CH2) in the stack, and the first storey and the second storey each constituting a chemoresistor type gas sensing element; and an activation unit for controlling activation of the sensing layer of the second storey; wherein the housing and the stack of chemoresistor type gas-sensing elements (CH1,CH2) define, for gas undergoing analysis, a gas flow path passing through the sensing layer of the second storey before reaching the sensing layer of the first storey whereby, in use, the composition of gas reaching the first storey is influenced by the presence and operation of the sensing layer of the second storey; and wherein there is further provided control means adapted to take measurements via the measurement electrodes of the first storey at times when the second sensing layer of the second storey is activated and de-activated, respectively, and to process said measurements, as well as an electrical signal from measurement electrodes of the second storey, to discriminate gases presented to the gas sensor.