The photoluminescence, PL, intensity from powedered CdS:Te phosphor (similar to 450 ppm Te), pCdS:Te reversibly changes relative to nitrogen as a reference gas upon adsorption of sulfur dioxide, ammonia, oxygen, and moisture. Specifically, we have found that the PL of the pCdS:Te reversibly increases by as much as 60% when exposed to ammonia and decreases by 50% when exposed to sulfur dioxide. The PL intensity also decreases to a lesser degree when exposed to oxygen and moisture. Ar, He, H-2, N2O, CO, and CO2 gave no response or a response of less than a few percent. The changes in PL intensity are concentration dependent and at partial pressures below 0.2 atm follow the Langmuir adsorption isotherm model. At higher partial pressures there appear to be more complicated adsorption processes taking place that are discussed. Adduct formation constants derived from the Langmuir adsorption isotherm model at pressures below 0.2 atm range from 20 to 35 atm(-1) for sulfur dioxide and 25-50 atm(-1) for ammonia at room temperature. pCdS:Te shows promise as an optically coupled chemical sensor for incorporation into gas sensor arrays.