A movable quadrupole mass spectrometer was used to measure the intensity and the beam profile of the As effusion cells in a molecular beam epitaxy (MBE) system for GaAs and AlGaAs. By measuring As-controlled reflection high-energy electron diffraction (RHEED) intensity oscillations, the mass spectrometer reading was calibrated in terms of the As flux. If the cells were driven as usual by keeping the cell temperature constant, the As flux showed unsatisfactory long-term constancy, and a strong dependence on the filling height. Equilibration of the desired As flux required to 1–2 h if the cell was heated from room temperature. This time was reduced to 30–60 min if the temperature setting was changed in the range of +-20°C. Control of the As emission with a quadrupole mass spectrometer yielded a stable As flux with deviations of less than +-4% during the whole growth process. As equilibration of the desired As flux needed only 10 min, reproducible changes of the As flux allowed growth of films with modulated As surplus in much shorter times. Thus, as an additional benefit, the total As consumption was lowered.