The electrosorption of 1,5-pentanediol (PD) on a Ag(111) single-crystal electrode surface was investigated from aqueous 0.05 M KClO4 on the basis of chronocoulometric measurements of the capacitive charge sigma(M) and of differential capacity measurements. The sluggishness of the PD adsorption/desorption step prevented differential capacity measurements under conditions of adsorption equilibrium, other than in the neighbourhood of the potential of maximum adsorption. Chronocoulometric measurements do not suffer from this limitation. Adsorption isotherms at constant potential E and at constant sigma(M) were therefore determined on the basis of a thermodynamic analysis of chronocoulometric sigma(M) versus E plots at different PD concentrations. The potential dependence of the electrosorption valency was found to be determined mainly by the reorientation of the water dipoles under the interfacial electric field. The adsorption isotherm at the potential of maximum adsorption. E-m, obtained by the thermodynamic procedure, was compared with that obtained from differential capacity data via an extra-thermodynamic procedure based on the Frumkin two-parallel capacitors model. The latter procedure was also adopted to estimate the adsorption isotherm of PD on Ag(110) at the potential of maximum adsorption. The absolute value of the standard Gibbs energy of adsorption at E-m decreases in passing from Ag(111) to Ag(110), namely with an increase in the hydrophilicity of the face.