The authors have determined the dielectric functions of a series of molecular-beam-epitaxy-grown (In0.5Ga0.5)(1-x)MnxAs thin films deposited on InP substrates. Two variable angle spectroscopic ellipsometers, covering both the IR and the UV range (0.2-30 mu m), were used to obtain optical spectra for each of the samples. Using a standard inversion technique, the experimental data were modeled to obtain the dielectric function for each of the quaternary samples. By using a parametric semiconductor model, they deduced the critical point parameters corresponding to the electronic transitions in the Brillouin zone. Their analysis indicates that in this particular quaternary system, while the critical point associated with the fundamental gap, E-0, blueshifts as a function of Mn concentration, the El critical point shows a redshift with respect to the Mn concentration. (c) 2007 American Vacuum Society.