The study of the interactions of cations with humic substances has been going on for half a century or more, and given the wide and continuing interest in the subject it is perhaps surprising that there has not been a text dedicated to the subject. With this volume, I attempt to fill the gap. My aims are to provide a basic account of the interactions, discuss how they can be represented in models, and relate the results to the natural environment. I hope the book will be of interest and use to students of environmental chemistry, to individuals beginning research projects, and to established research workers, not only chemists but also those in other environmental disciplines.

I am grateful to Peter Campbell (INRS-Eau, Canada), who first suggested that I write this book, and to John Hamilton-Taylor (University of Lancaster) who kindly read and commented upon draft chapters. Lisa Baldwin's cheerful help in obtaining references, maintaining my filing system, and obtaining permissions to reproduce published figures, is greatly appreciated. Thanks are due to James Alberts, David Kinniburgh and Erwin Temminghoff for supplying copies of original diagrams. The present copyright laws require permission to be obtained only from the publisher of a work to reproduce figures and tables (although some publishers require the author(s) to agree as well). However, I sought the approval of all the authors anyway, and was much heartened to receive the willing agreement of all who replied, and by the many good wishes expressed.