The chemical dissolution of porous silicon (PS) layers hydrofluoric (HF) acid solutions of various compositions has been investigated. It is shown that it is possible to determine the specific surface area of PS from comparison of bulk Si and PS etch rates. We have used this new technique to characterize PS layers obtained from lightly doped p-type Si. It is shown, for the first time, that the specific surface area significanfly decreases when the porosity of the material increases. This surface area decrease is discussed in relation to the photoluminescence (PL) properties of the porous layer. Furthermore, the etch rate of PS, which increases with decreasing HF concentration, is found to correlate more with the pH of the solution rather than the HF concentration.