Optical losses measurements were performed at 1550 nm on both porous silicon (PSi) and oxidised PSi (OPSi) planar waveguides as a func- tion of interface roughness. The PSi waveguides were produced at different anodisation temperatures to reduce the interface roughness. The authors noticed a decrease in optical losses with the diminution of roughness. The lowest value of optical losses was equal to 0.5 dB/cm and obtained for OPSi waveguide with a 10 nm interface roughness and anodisation temperature equal to 2208C. The surface scattering losses were modelled. There is a good agreement between the theoretical and experimental values. The optical losses were also carried out in trans- verse electric (TE) and transverse magnetic (TM) polarisations for both PSi and oxidised OPSi waveguides.