The optical and structural properties of a porous GaAs have been studied. The samples a of porous GaAs were fabricated by an electrochemical method on n- and p-type GaAs(100). The GaAs wafer doping type considerably affects nanocrystal shape, nanocrystal average diameter and chemical surface states. Low-frequency Raman shift of the peaks, conditioned by the main optical phonons, in the Raman spectra of the porous GaAs was observed. The values of the frequencies of surface phonons obtained from the Raman spectra and Infrared reflectivity spectra well coincide. Comparing the reflectivity spectra of porous GaAs with the ones of the single crystal GaAs, the changes in the spectral dependencies of the reflectance within the phonon resonance region may be seen which coincide with appearance of additional oscillators caused by sized confinement of the lattice vibrations of GaAs nanocrystals. The surface morphology of porous GaAs prepared on the substrate of n-type GaAs has been studied using atomic-force microscopy. Nanosized contour of the porous GaAs surface was watched. Estimations of the size of nanocrystals in a porous GaAs by the Raman and Infrared spectroscopy, photoluminescence (PL) and atomic-force microscopy well agree with each other. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)