Mesoporous GaAs double layers with different porosities and thicknesses up to 7 µm are formed on highly doped p-type 4 inch GaAs substrates by means of electrochemical etching in highly concentrated hydrofluoric acid electrolytes. Small 〈111〉 oriented pyramids form at the interface between porous layer and substrate if etching current densities below 7.5 mA cm−2 are applied. Porous double layers with different porosities are obtained by varying simultaneously the etching current density and the electrolyte concentration. The porosity of the porous GaAs layers decreases nearly linearly from 69 to 53% with electrolyte concentrations increasing from 30 to 50 wt.%, respectively. The etching process increases the mean roughness of the porous layer surface from 0.15 to 0.24 nm.