The optical properties of In0.52(AlxGa 1-x)0.48As epilayers with various x values were systematically studied using variable angle spectroscopic ellipsometry in the wavelength range of 310-1700 nm. The refractive indexes were determined and could be given as n(x)=0.12x2-0.51x+3.6 at the wavelength of 1.55 μm. The measured thickness of the epilayers agrees within 5.2% of the nominal thickness. The energies and broadening parameters of the E1 and E1+Δ1 transitions as a function of Al composition were also examined based on the second-derivative spectra of the dielectric function. The comparison between the results and the reported data is presented.