Electron spin resonance (ESR) parameters (free radical concentrations, g-values and line widths) were determined on powders and solutions of fulvic acid (FA) and on a number of different molecular-weight fractions separated from it. Major objectives were to evaluate effects of pH, reaction time, chemical reduction and irradiation on the ESR parameters and to obtain information on the identity of the free radicals. Two types of free radicals were detected in all FA-preparations: (a) permanent ones, having long life spans; and (b) transient ones, with relatively short lives, which were generated in large concentrations by different treatments in the following order of decreasing efficiencies: chemical reduction > irradiation > raising the pH. Spectroscopic splitting factors (g-values) of permanent and transient radicals were similar, indicating that the radicals had similar structures. From the magnitude of the g-values it was likely that the radicals were substituted semiquinones which, in alkaline solutions, were stabilized as semiquinone ions. In most cases, the treatments that generated transient radicals were reversible. While the permanent free radicals were stabilized by the FA-structure, the transient ones were not. Reasons for this behavior remain to be uncovered. The generation from FA of large concentrations of transient free radicals by chemical reduction and irradiation should be of interest to soil scientists in that it might have a number of practical applications.