The oxidation state of structural iron greatly influences the physical-chemical properties of clay minerals, a phenomenon that may have significant implications for pollutant fate in the environment, for agricultural productivity, and for industrial uses of clays. Knowledge of redox mechanisms is fundamental to understanding the underlying basis for iron's effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this variation may not have been a simple function of the reduction potential of the reducing agent. The objective of the study was to identify the relationship between the Fe reduction mechanism and the free radical activity in the reducing agent. Several reducing agents and their mixtures with the Na-saturated, 0.5 to 2 micrometers size fraction of ferruginous smectite (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. (Copyright (c) 1992, The Clay Minerals Society.)