A sensor has been developed for the automatic identification of several categories of materials found in municipal solid waste. The principle of the sensor is to identify materials on the basis of reflected infrared radiation of specific wavelengths. Materials which can be identified with near 100% reliability include cellulose products (primarily paper), plastics, metals, and (with less reliability) glass. More detailed classification is possible in some circumstances. The sensor has been incorporated into an automated refuse separation pilot plant developed at MIT. Results of a related study of the physical characteristics of actual municipal refuse are also reported. (Modified author abstract)