Density and sound velocity of aq. solns. contg. monofunctional primary, secondary and tertiary amines, as well as bifunctional primary and secondary amines (B), and their corresponding hydrochlorides, (BHCl), were measured; the apparent molar compressibilities were calculated.  From the limiting apparent molar compressibilities of B, BHCl and HCl in aq. soln. the changes in compressibilities, Δϕ°K,S, for protonation of B was obtained.  The pattern shown by the Δϕ°K,S values was similar to that presented by the changes, ΔX°r, of other thermodn. properties, such as Cp and V, for the formation of BHCl species from B and HCl.  The different values of Δϕ°K,S obsd. for amines having the same no. of hydrogens bonded to N but differing in the presence of a second hydrophilic group was attributed to the removal of the hydrophobic hydration of the alkyl groups brought about by this center.