Spectroscopic ellipsometry (SE), a non-destructive optical method, was used for the characterization of In0.48Ga0.52P-based thin-layer structures. Two types of sample structures were investigated. One was an In(0.48)Gao(0.52)P-based single-layer structure and the other was an In(0.48)G(0.52)P-based multi-layer structure. Our objective was to determine the optical properties of In(0.48)Gao(0.52)P for the single-layer samples and to characterize the structures of the multi-layer samples, such as layer thicknesses. For each sample investigated, the measured spectra were analyzed with an appropriate fitting model, which was constructed based on the sample’s nominal structure. In the determination of the optical properties of In(0.48)G(0.52)P, two methods were employed, namely, the harmonic oscillator approximation and the wavelength-by-wavelength fitting method, which is routinely used in SE analysis, The advantages and disadvantages of these two methods are discussed with the results from the chosen samples. In the structural characterization, the thickness results obtained by SE were compared with the corresponding ones determined by cross-sectional transmission electron microscopy. It is shown that, for each sample under study, very good agreement was found between the results obtained by the two methods. This in turn demonstrates that SE is reliable for thickness characterization of the multi-layer samples.