Sub-micrometre area porous silicon structures were fabricated by anodization of patterned surfaces of crystalline p-type silicon, orientation (100) and resistivity 5-7 Omega cm. The mask for selective anodization was either silicon dioxide or silicon nitride, deposited onto silicon by low pressure chemical vapour deposition. The second mask was more resistant to the anodization solution (HF, ethanol and water solution) and was not dissolved during the anodization time used. When silicon dioxide was used as a mask, carl was taken to stop anodization before the whole layer was removed. Patterning was done by optical lithography combined with a special process for sub-micrometre dimensions (silylation process). Dots and lines of dimensions down to 0.5 mu m were produced, which exhibit strong red luminescence under UV excitation. The film thickness of the porous silicon structures in the sub-micrometre areas was smaller than the thickness in larger areas. The lateral growth of the structures under the mask was examined by scanning and transmission electron microscopy. Microphotoluminescence experiments in a confocal microscope were also used for mapping the luminescent areas.