The carboxyl content of different fulvic acids was estimated by means of a methylation technique. The procedure involved methylation by diazomethane followed by base catalyzed hydrolysis, esterification with propionyl chloride and gas chromatographic analysis of the methyl ester formed. After optimization of each step, the whole sequence of reactions was performed using only a few milligrams of starting material. The method was applied to two fulvic acid samples of different origin (surface water and groundwater, respectively). The carboxylic content was estimated to about 3.5 meq/g material for the surface water fulvic acid and about 4.5 meq/g material for the groundwater fulvic acid. This estimates approximately 80% of the total content of acidic groups obtained for the same materials by using potentiometric titration.