The structure of lidocaine hydrochloride (LidHCl) in propylene glycol (PG), a solvent known to enhance transdermal delivery of drugs, and the mobilities of the different kinds of ionic species appearing in this system was investigated at 25.0°C by precision conductometry. The molar conductivity was determined at several concentration between 0.4 and 10 mM and the data analysed using the conductance equation of Fuoss-Hsia and Fernandez-Prini (FHFP equation). For concentrations of up to 1.2 mM no higher aggregates that LidH + were found. Using a two-parameter analysis of the equilibrium, LidH + Lid+H + , we obtained the acid dissociation constant, K a (LidH + )=2.5 10 - 7 (molar scale); pK a =6.60, and the limiting molar conductivity, λ 0 (LidH + )=0.2675 cm 2 /Ω per mol. For concentrations above 1.2 mM there is strong evidence of formation of ion-pairs, LidH + Cl - . The ion-pair association constant was estimated to K p 40 indicating that about 15% of the electrolyte is in the form of LidH + Cl - at the highest concentration (10 mM) investigated.