This paper first gives a brief review of the main techniques used to measure the lipophilicity of neutral and ionic drugs, namely the shake-flask method, potentiometry, and cyclic voltammetry at liquid-liquid interfaces. The lipophilicity of 28 acidic compounds with various functional groups was studied by potentiometry and cyclic voltammetry in the n-octanol/water and 1,2-dichloroethane/water systems in order to complement our understanding of the lipophilicity of neutral and ionized acids and to clarify the solvation mechanisms responsible for their partition. The parameter diff (log P(N-A)(dce)) (i.e., log P of the neutral acid minus standard log P of the conjugated anion in 1,2-dichloroethane/water) was shown to depend not only on intramolecular interactions and conformational effects in the neutral and anionic forms, but also on the delocalization of the negative charge in the anion, confirming the ability of Born's solvation model to describe qualitatively the effect of the molecular radius on the lipophilicity of ions.