Hg1-xCdxTe is the leading material for high-performance long-wavelength (lambda similar to 10-20 mum) infrared detectors. At these wavelengths, highly accurate compositional control (Deltax less than or equal to 0.002) is required to achieve a particular device cutoff and the detector's performance characteristics. Spectroscopic ellipsometry has proven to be a highly accurate technique of measuring the HgCdTe composition during epitaxial growth. Here we present the growth of HgCdTe by molecular beam epitaxy using an automated control program using real-time feedback from spectroscopic ellipsometry measurements. Excellent control is demonstrated for more than 50 growth runs with a standard deviation of Deltax = 0.0004 observed for the error between the composition measured by ellipsometry and the target growth composition.