Combined reflectance R and reflectance anisotropy spectroscopy (RAS) was applied for in situ monitoring of composition and growth rate of MOVPE grown In1-xGaxAs1-yPy layers lattice matched to InP. The sum of the surface sensitive RAS signals at 1.75 and 2.65 eV associated with the phosphorus to arsenic ratio was used to evaluate the composition parameters x and y calibrated by ex situ measurements. For fixed MOVPE growth conditions, the ratio of the growth rates of InGaAsP and InP growth rate, calculated from the reflectance signals, directly corresponds to the gallium-to-indium ratio of the InGaAsP-layers. (C) 2004 Elsevier B.V. All rights reserved.