The activity coefficients (and therefore the activities) of single ionic species are concepts tentatively introduced by G. N. Lewis, which he could not define thermodynamically because of electroneutrality requirements. Guggenheim subsequently gave their formal, pseudo-thermodynamic definitions, while warning that they were imaginary constructs without physical significance. Consequently, the hydrogen ionic activity, as a purely conceptual but immeasurable quantity, cannot serve as the basis of the pH, the globally accepted experimental measure of acidity. Various aspects of this mismatch are described, based on the original literature sources as well as on experimental data used by their proponents. An especially pernicious but apparently widespread misconception is that the hydrogen ion concentration cannot be determined by thermodynamic means, despite the extensive work of Harned, Robinson and coworkers who showed otherwise. A pathway is indicated to facilitate a smooth return to the original, thermodynamically sound definition of Sorensen in terms of the hydrogen ion concentration We also describe a useful formalism based on the imaginary nature of single ionic activities. (C) 2014 Elsevier Ltd. All rights reserved.