The Special Sensor Microwave/Imager (SSM/I) flying on the most recent satellite of the Defense Meteorological Satellite Program (DMSP) measures radiation from the Earth-atmosphere system within millimetre wavelengths. This radiation is strongly modified by the varying atmospheric water-vapour content. A technique to derive the water-vapour content over oceans from these measurements is developed. The sensor system is studied theoretically by calculating the radiative transfer for a non-scattering atmosphere in the appropriate spectral channels for a large set of atmospheric/oceanic situations. Subsequently, statistical retrieval coefficients are derived via a regression analysis of the synthetic measurements. Approaches using one to three spectral channels enable us to derive water vapour amounts in vertical columns with accuracies from 0.145 to 0.17g/cm². A case study with real SSM/I data demonstrates the capability of the system to derive atmospheric water-vapour fields over the global ocean within a period of a few days. The accuracy evaluated is confirmed through globally distributed match-ups with radiosonde measurements.