It is well known that on the basis of strictly thermodynamic arguments single ion activities are not amenable for direct experimental determination [1–3]. However, single ion activities are much more significant than mean activities with respect to the problem of assessing the structure of the solutions. Therefore, various methods have been proposed to derive single ion activities from experimental quantities. All these methods necessarily imply some non-thermodynamic arguments and approximations. The unavoidable step to be taken to at single ion activities is thus the introduction of some model assumptions. Experimental methods have been proposed based on (a) the correction for the liquid junction potential of measured emf's of cells with constant potential reference electrodes [4], (b) the measurement of the potential difference for a non-isothermal system constituted by two equal electrodes [5], and (c) the measurement of the potential difference for an ‘air gap’ cell [6] of the type Pt¦H 2¦HCl( m)¦air¦HCl ( m ref¦H 2¦Pt. A proposed non-experimental route rests [7] on the assumption that for not very dissimilar ions the difference in activity coefficient γ i is essentially related to the difference in ion solvation. A number of factors which may make γ + to differ from γ − have been discussed by Frank [8]. The ultimate scope of a measurement to determine single ion activity coefficients is possibly to reproduce the condition of constancy in potential of one of the two electrodes in an electrochemical system. In this work we propose to adopt the electrical double layer model in the absence of specific adsorption [9] as the one best meeting the above requirement. This model has been verified independently [10] and within the validity of the usually accepted Stern-Gouy-Grahame theory, it can be used without any further justification. Work in this direction was first carried out by Stastny and Strafelda [11]. No further attempt appears to have been made. The basic assumption is that a cell of the type (σ is the charge density on the metal): Hg(σ = const)¦NaF( m)F − selective electrode, (1) exhibits a potential difference strictly dependent on the sole activity of F −. Although the method is in principle valid at any value of σ, the direct measurement of E for cell (1) can be carried out only at the potential of zero charge. E at other values of charge can be obtained by integrating double layer capacity data and correcting for diffuse layer effects (concentration effects at the Hg electrode). Capacities and potentials of zero charge have been determined for Hg in a number of NaF aqueous solutions at concentrations ranging 0.01 to 0.9 mol dm −3. An Orion LaF 3 crystal electrode has been used as the F − sensitive electrode. From these measurements the γ − values have been derived with the procedure described elsewhere [12]. The γ + values have been obtained from the known values of the γ ± for NaF [13]. γ − have been found to be higher than γ +'s. This will be shown to be in disagreement with the prediction of the ‘hydration’ theory [7]. The values of γ − are apparently independent of the value of σ on Hg. Ultimately, these results point to some reciprocal support of the various theories involved in the present approach. The present results will be compared with those obtained with the ‘air gap’ cell [6]. Merits and limits of the present method will be discussed.