The adsorption of 2,2-dimethyl-1-propanol (neopentyl alcohol, NPA) on a Ag(111) single-crystal electrode surface from aqueous 0.05 M KClO4 as a function of the applied potential E was investigated on the basis of measurements of the capacitive charge sigma(M) by a computerized chronocoulometric apparatus and of the differential capacity C by a frequency response analyzer. A satisfactory agreement was found between directly measured C vs E curves and those obtained by numerical differentiation of the corresponding sigma(M) vs E curves. Adsorption isotherms at constant E and at constant sigma(M) were determined on the basis of a thermodynamic analysis of capacitive charge data. The standard Gibbs energy of adsorption, Delta G degrees (ads), was determined as a function of E and of sigma(M), and attains a minimum value of -12.6 kJ mol(-1) at E = -0.77 V/SCE and sigma(M) = -1.1 mu C cm(-2). The results of the thermodynamic analysis of adsorption data at the potential of maximum adsorption were compared with those of an extrathermodynamic analysis based on the Frumkin two-parallel capacitors model.