GaAs epitaxially grown on porous Si (PS) showed the presence of wires with typical diameters of 60 nm, and lengths in the range 6-10 . The Raman spectra of these samples were studied, and we focused attention on two additional modes appearing as `shoulders' to the LO and TO GaAs lines. The former could be assigned to a surface phonon, and the latter to the presence of As clusters.