A qualitative physical characterization of particulates emitted in undiluted exhaust of single cylinder direct injection (D. I.) diesel engine was performed. In the experiments, an optical technique based on a Xenon lamp was used to measure scattering and extinction coefficients in the spectral range of 0.2 to 0.6 nm at the exhaust of an engine operating under steady-state conditions. The spectral nature of the data was first compared against the theoretical predictions performed for spherical particles and clusters, using the complex index of refraction data for glassy carbon. Additional calculations were performed after determining the spectral refractive index of exhaust soot directly from experiments, which yielded better comparisons with the measured scattering efficiency spectra. The discrete dipole approximation was employed to predict radiative properties of 16, 32, and 64-sphere random-shaped clusters. Based on these comparisons, the effective size of exhaust soot particles was estimated qualitatively for different experimental conditions