A complete system to obtain ratios of stable isotopes of certain elements (carbon and oxygen from carbonates, sulfur from sulfides and sulfates) is described. The chemical preparations necessary to convert the geological samples to a suitable gas (CO2or SO2) are discussed. The isotopic measurements are performed with a modified Micromass 602C mass spectrometer. The computer programs used for corrections and calculations are also discussed. Stress is laid on those parts of the above procedures where we have modified earlier methods. The system has been in continuous use for about four years. A survey of geological projects partly based on results obtained with this system is given in the last section.