Porous silicon (PS) layers are formed on p(+)-type silicon wafers by electrochemical anodization in hydrofluoric acid solutions. Microechography and acoustic signature, V(z), have been performed at 1.5 GHz and 600 MHz, respectively, in order to study the elastic properties of PS layers. The thicknesses of PS layers were measured and longitudinal, shear and Rayleigh velocities and Young’s modulus were obtained as a function of porosity. Equations showing the porosity dependence of bulk wave velocities and Young’s modulus have also been proposed.