Real-time composition control using spectral (or spectroscopic) ellipsometry (SE) in the growth of long-wavelength infrared (LWIR) Hg1-xCdxTe (x similar to 0.225) on Cd0.96Zn0.04Te(211) B by molecular beam epitaxy (MBE) was investigated. Excellent compositional reproducibility among the 10 LWIR Hg1-xCdxTe growth runs was demonstrated with the aid of SE, with the average composition being x = 0,225 acid the standard deviation in x being 0.00042, the lowest figure that has ever been reported. The ability of MBE to switch from one composition to another on demand and with first pass success using SE is also demonstrated.