The influence of composition changes in (AlxGa1-x)(1-y)InyP grown on GaAs in metal-organic vapour-phase epitaxy on the in situ measured optical signals reflectance anisotropy (RA) and normalized reflectance (NR) is studied. The aluminium composition x (band gap) is changed from 0 to 1 while the indium composition y (lattice matching) is adjusted about +/- 0.03 around the lattice matched value of y = 0.48. The optical signals were correlated to the composition which was determined by ex situ measurements. A strong influence of both compositions x and y, respectively, is observed in the RA and the NR spectra and transients. The composition changes have an influence on the amplitude as well as on the energetic positions of the respective features in the RA spectra. A strong influence of the composition on the first amplitude of the UV Fabry-Perot interference in the time-resolved NR signal can be seen. Very thin layers, 12 nm, are sufficient for the in situ determination of the composition. The results are used for in situ composition calibration procedures. (c) 2005 Published by Elsevier B.V.