Variations of smoke aerosol microstructure at the stages immediately following thermal decomposition of hydrocarbon compounds are analyzed based on laboratory measurements of light-scattering parameters with a polarization spectral nephelometer. Peculiarities of variations of the disperse composition and optical constants of aerosol particles at different modes of their formation are considered through direct modeling of aerosol scattering parameters and solution of the inverse problem. Estimates show that at different stages of wood smoke evolution, several maxima are observed in the size spectrum at particle radii of 0.08, 0.25, and 0.45 mkm. The imaginary part of the refractive index of particles did not exceed 0.004, and the real part varied from 1.48 to 1.57. The increase of the pyrolysis temperature resulted in significant burning-out of the coarsest fraction and increase of the real and, especially, imaginary parts of the refractive index up to 0.05.