Abstract: The dew points of condensible hydrocarbon and water in a gas stream are determined independently in a single run by observing changes in intensity of light scattered from a mirror surface which is cooled to below both of the dew points. A decrease in intensity denotes the hydrocarbon dew point while an increase in intensity denotes the water dew point. The apparatus is simple and operates automatically. The mirror surface has a first polished and highly reflective portion and a second, roughened, less reflective portion. The intensity of scattered light is preferably monitored separately for each mirror portion.