The use of in situ quadrupole mass spectrometry (QMS) provides the precision necessary for many molecular beam epitaxy (MBE) applications in which the composition of ternary or quaternary epitaxial layers must be controlled. Here we show that the composition of InxGa1-xAs, nearly lattice-matched to InP, can be determined with an uncertainty of Delta x similar to 0.005 in real time (similar to 4 s averaging time) using QMS in a flux-monitoring mode. Use of an unapertured mass spectrometer extends the range of reflection mass spectrometry into low substrate temperature growth regimes. We also examined system calibration and drift issues. QMS combines routine data acquisition with simple data analysis, and can be easily retrofit on existing MBE systems.