A process is presented based on anodization of silicon in the transition region between the porous silicon formation regime and the electropolishing regime. Bright and stable photoluminescence was obtained on p(-), n(-) and p(+), n(+) (even degenerate) silicon. We report the photoluminescence, IR absorbance and thermal diffusivity of porous silicon formed by anodizing in the transition regime the four types of silicon. Fourier transform IR and gravimetric investigations showed that anodic silicon suboxide is formed on the surface. A model is proposed for the porous structure, which is suggested to consist of silicon crystallites built inside an anodic oxide.