A new multi-channel temperature and humidity monitor has been developed. The sensor is a bifunctional and composite device consisting of temperature- and humidity-sensitive components which are made of semiconducting ceramics. A single-chip MCS-51 computer collects and processes the sensor data and sends out control and alarm signals when limits preset by the user are exceeded. This paper describes the principle and characteristics of the bifunctional sensor, the operation of the detector circuits, and the hardware and software of the single-chip computer.