In this review, we will examine the problems related to the chemisorption of molecules on electrodes, where the adsorbate may possibly share its electrons with the metal. The "partial charge transfer" between adsorbate and electrode cannot be measured, since the division of the bonding electrons into parts pertaining to the adsorbate and to the electrode is arbitrary; at most, it can be tentatively estimated on the basis of modelistic considerations. This point will be clearly demonstrated on the basis of simple thermodynamic considerations. A thermodynamically significant and experimentally measurable quantity that has often been conceptually related to partial charge transfer is the "electrosorption valency". We will discuss methods by which it can be measured, both when only a single species is adsorbed and when its adsorption is accompanied by coadsorption or competitive adsorption with other species. A simple unified modelistic picture will be adopted to illustrate the extra-thermodynamic partial charge transfer coefficient for low surface coverages of ionic and neutral adsorbates and for compact chemisorbed monolayers both in solution and in vacuum. The relation between partial charge transfer coefficient and the dipole moment of the chemisorption bond will be clarified and the estimate of this bond in vacuum by work function or outer potential measurements will be described. (C) 2014 Elsevier Ltd. All rights reserved.