Characterization of porous silicon (PS) on the atomic scale is difficult and optical techniques offer certain advantages. High vacuum studies of PS by optical second-harmonic generation (SHG) are presented. Excitation wavelengths corresponding to a surface state and a suspected strain resonance from clean Si surfaces are used. Hydrogen-terminated and clean surfaces of Si(100) and PS are compared. The H-terminated surfaces give no signal under the excitation conditions used. On desorbing the hydrogen, a stable SH signal is obtained from the PS which has a polarization behaviour consistent with a significant contribution from surfaces of {111} orientation.