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Single-layer (porosity 64 % and 48 %) and double-layer porous silicon (PS) samples about 5 µm thick, formed by electrochemical etching of p$^+$-Si(100) at 200 and 50 mA/cm$^2$, were studied before and after thermal annealing in air. Spectroscopic ellipsometry Ψ(λ), Δ(λ) spectra were analysed in the transparent near-infrared region using a sequence of models: a homogeneous Cauchy layer, two- and three-component (Si + SiO2 + voids) Bruggeman effective-medium approximations, and a layer with a depth gradient of the refractive index. The obtained thicknesses agree with scanning electron microscopy, and the porosities agree with gravimetry. In the double-layer sample, the porosities of the two layers (71 % and 52 %) match those of the single-layer samples etched at the same current densities. The layer etched at 200 mA/cm$^2$ is nearly homogeneous in depth, whereas the layer etched at 50 mA/cm$^2$ shows a pronounced inhomogeneity that persists after annealing. Before annealing, the oxide is confined to a thin near-surface layer, as confirmed by EDS oxygen profiles. After annealing, the oxygen content increases markedly throughout the layers, the layer thickness increases and the refractive index decreases. The highly porous layer oxidizes completely and contains ≈ 56 % SiO2 while retaining a large fraction of pores, whereas in the less porous layer the oxide fills nearly the entire pore volume. A volume-balance analysis that accounts for the volume expansion upon Si → SiO2 conversion ($\kappa \approx 2.27$) shows that the amount of silicon is conserved and that the degree of oxidation is governed by the initial porosity and layer expansion. The approach enables non-destructive evaluation of porosity, oxide fraction, oxidation degree and inhomogeneity of individual layers in PS multilayers.
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