Abstract: In apparatus for detecting or identifying gas or liquid or for measuring their concentration, an array of sensors arranged for simultaneous exposure to a substance is coupled to electrical circuitry arranged to generate electrical signal values which are linearly related to substance concentration. The sensors are selected to respond differently to a known sample substance or substances. By feeding the signal values to a processor programmed to combine the signal value in a series of linear equations, a series of output values are obtained which are characteristic of the sensed substance or substances which, when compared with stored data allow identification of the sensed substance or substances and the measurement of substance concentration.