A novel method for identification of chemical compounds using multiple parameters is proposed. An ac voltage was applied to a titanium dioxide (TiO2) semiconductor gas sensor. The resulting conductance, surface potential and phase lag of the output current wave form with respect to the input voltage wave form in the dark and under illumination were recorded. It was found that the sensitivity of the sensor increases almost one order of magnitude under a monochromatic light (at 700 nm) intensity of 50 mW cm(-2). Three dimensional figures representing these three linear parameters enabled us to distinguish alcohol and benzene compounds at low concentration, with a single detector.