We have studied optical losses as a function of the wavelength for planar waveguides formed from porous silicon or oxidized porous silicon. Scattered light from the surface of samples was also observed. This observation reveals the influence of porous silicon dissolution front fluctuations called waviness on propagation. After oxidation, the measured losses decreased strongly and attained a value equal to 0.5 dB/cm in the near infrared. Surface and volume scattering losses were modeled in order to determine their principal contributions to overall losses. For porous silicon waveguides obtained from a P+ silicon substrate, the losses were mainly due to absorption by the material; whereas, for oxidized porous silicon waveguides, the principal contribution depends on the used wavelength. In the visible spectrum, losses due to volume scattering were predominant while in the near infrared, surface scattering was responsible for most of the losses.