An amperometric electrochemical sensor for measuring the concentrations of two or more target gas species in a gas sample or gas stream, wherein the sensor includes first and second electrochemical cells having respective first and second active electrodes, the electrochemical cells further including an electrolyte membrane and a counter electrode, wherein the first electrochemical cell exhibits an additive response with respect to a first and second ones of the target gas species and the second electrochemical cell exhibits a selective response to the first target gas species in the presence of the second target gas species such that the sensor is capable of measuring the respective concentrations of the first and second target gas species.