A systematic investigation of the ellipsometric parameters of MBE-grown heterostructures of ln(x)Ga(1-x)As on GaAs substrate has been completed. The index of refraction n, and extinction coefficient, k, values of the above heterostructure in the wavelength range 500-800 nm, are presented, a region of interest in many applications. A model has been proposed for the multilayered structures, through which the thickness of the oxide layer can be determined and the observed optical characteristics of these heterostructures explained. The validity of the model was established by the excellent agreement between the measured end calculated values of the ellipsometric parameters psi and Delta.