The mass spectrometer is capable of making rapid, repetitive multicomponent analyses with measurement times as short as a second or less. Mass spectrometers used to be too expensive to be used for process analyses, but simpler and cheaper versions have been developed during the last decade. Mass spectrometers are commonly used within the petrochemical industry, but their use for fuel gas streams is not so widespread. This paper describes tests on a quadrupole mass spectrometer in three different gasification projects. Calibration methods and analysis accuracies obtained are accounted for. On the basis of the tests, it can be concluded that the quadrupole mass spectrometer is very suitable for continuous analysis of complex process streams. An important result is that the separation of CO and N-2, an analysis problem that is traditionally regarded as difficult, worked very well. The analysis frequency was approximately once per minute, which should be enough for most control loops.