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A highly sensitive gas sensor for detection and analytical identification of hazardous gases was developed, by depositing partially crystalline powder from an aqueous suspension of CdS powder on an alumina substrate. When the deposited layers were promoted with catalysts from Pt-group metals, they exhibited high sensitivity to different gas molecules upon modulation with light of certain frequency, depending on catalyst and impurity concentration. Dark resistance and photosensitivity of the sensors were monitored against gas concentration, sensor temperature, and frequency of the modulating light. Depending on the gas and its concentration, a change of up to 100% in sensor response was observed. Variation of photosensitivity with dark resistance showed different slopes for different molecules. Variation of the low frequency phase angle with photosensitivity at different concentrations also produced different slopes, depending on the type of molecule. This differentiation likely comes about by different interactions of gas molecules with surface recombination centers, depending on the nature of physisorption and/or chemisorption on the surface. Thus, in addition to detecting the gas, it can be identified from their slopes of variation with light frequency, and the sensor can be used as an analytical tool. © 1997 American Institute of Physics.