The Tafel slope serves as an important diagnostic tool in the analysis of the mechanism of electrode reactions. It is common to assume a value of beta = 0.5 +/- 0.1 for the symmetry factor, although there is no solid theory to support this. Metal deposition represents a special class of reactions, in that charge is carried across the interface by the metal ion, not the electron. Although this has been stated in the literature, no proof was offered. This represents a physical situation that is distinctly different from outer-sphere electron transfer, and there is absolutely no justification to assume a priori any specific value for the symmetry factor. In the current paper and in several earlier publications of the present author, this issue has been discussed in detail, proving that in metal deposition charge is carried across the interface by the metal ion, not by electrons. A mechanism is proposed here, based on the assumption that the ions migrate across the double layer under the influence of the high electrostatic field created by the applied overpotential. This mechanism can also explain the observation that the heterogeneous rate constants for metal deposition are often higher or comparable to those of outer-sphere electron transfer reactions, in spite of the fact that in the case of metal deposition bonds are broken to remove the hydration shell, while outer-sphere electron transfer does not require the breaking of any bonds. The significance of the symmetry factor in the framework of this model is discussed. It is shown that there is no basis to assume a priori any particular value of beta. A further situation, where an anion is adsorbed on the surface and transfer of charge is required only between the surface of the electrode and the inner Helmholtz plane, which is the distance of closest approach of the adsorbed anion is also discussed, and it is shown that the value of beta should be inherently less than 0.5. (C) 2011 Elsevier B.V. All rights reserved.