The Nernst-Planck mass-transport treatment is extended to electrolytic multicomponent systems including mixtures of strong electrolytes, weak electrolytes, and hydrolysis reactions, in dilute, stagnant solutions. The extension is presented in the form of theorems of electrochemical mass transport. The validity and usefulness of the theorems have been demonstrated in their application to two important and complex systems, namely, electrodialysis at larger than limiting currents, and corrosion of ferrous alloys in waters at high temperature and pressure. These applications are briefly summarized here. The theorems are useful for treatment of electrochemical mass transport in boundary layers that can be approximated by stagnant layers. The theorems should prove useful to workers and students of electrochemical transport problems, especially for problems involving electrolytic multicomponent and heterogeneous systems. (c) 2005 The Electrochemical Society.