We describe the design, the development, and the calibration of a radiometer to measure water vapor abundance in the middle atmosphere, using the solar occultation technique from an orbiting platform. The use of gas cells containing water vapor and carbon dioxide leads to a relatively simple, but sensitive, design. This radiometer is shown to be capable of detecting water vapor in the mesosphere, where, because of dissociation by the Sun, the amounts are very small, of the order of 1 part per 106or less. A flight version of the instrument will be launched in 1992 on board the European Retrievable Carrier space platform, and the data will be employed for a better understanding of the Earth’s water budget.