The paper presents a comparison of x-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of and Si wafers. Whereas for the P-Si processed from Si the presence of some amorphous silicon is detected, the x-ray emission spectra of porous Si prepared from Si display a higher content of . For spark-processed Si the x-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10 000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the centre of the processed area is close to that of silicon dioxide, and depends slightly on the emission maximum. Specifically, green-photoluminescing sp-Si shows a higher degree of oxidation than the blue-luminescing specimen. However, the depth of oxidation consistently decreases for areas with weak photoluminescence and without photoluminescence. Possible origins for the photoluminescence are discussed.