A new three-step permethylation procedure for humic acid fractions has been developed. In the first step, carboxylic acid groups are methylated with diazomethane in dimethylformamide; in the second step, hydroxyl groups are methylated with methyl iodide and dimethyl sulfinyl carbanion in dimethylformamide; and in the third step, acidic groups hydrolyzed in the earlier steps are methylated with diazomethane. The resulting product is completely soluble in methylene chloride (a high degree of methylation has been achieved).