The hydroxyl group functionality of a series of aquatic humic and fulvic acids was examined by methylating the samples with ¹³C-enriched reagents and looking at the ¹³C NMR of the resulting methyl esters and ethers. The methylation procedure entailed methylation with ¹³C-diazomethane followed by permethylation with ¹³C-methyl iodide/sodium hydride, DMF the solvent in both steps. The ¹³C NMR of diazomethylated samples allows one to distinguish between carboxylic acid OH, phenolic OH, and phenolic OH adjacent to two substituents. The ¹³C NMR spectra of permethylated samples shows that a significant amount of the hydroxyl group functionality can be methylated only when a strong base is used. The sources of the aquatic humic substances investigated included groundwater, lake, bog, and swamp waters. The ¹³C NMR spectra of the permethylated humic and fulvic acids revealed that the total hydroxyl group pattern--carboxylic acid OH, carbohydrate OH, phenolic and aliphatic OH--was very similar among these aquatic samples despite the fact that they represented biogeochemically different sources. Practical applications of this ¹³C NMR characterization technique are briefly illustrated. These include following the chemical changes in the Spirit Lake fulvic acid following the eruption of Mt. St. Helens, analyzing the chromatographic fractionation of a groundwater fulvic acid, and analyzing the residual products after chlorination of two commercial humic acids. Side reactions of the methylation procedure were also studied. A ¹⁵N NMR investigation of the 1,3 dipolar addition of diazomethane to a soil fulvic acid led to the tentative conclusion that the cycloadduct was an N-methyl pyrazoline. Evidence is presented that carbon methylation occurs to a large extent during the CH₃I/NaH step. The derivatization of ketone and/or quinone groups with methoxylamine for the purpose of viewing these groups as the N-OCH₄ derivatives in the ¹³C NMR was also examined. Solid state CP/MAS ¹³C NMR spectra of aquatic fulvic acids are presented and discussed. Structural features evident in liquid state ¹³C NMR spectra of samples permethylated with unenriched reagents and examined at high field are also discussed. Carbonyls of aromatic acid methyl esters are distinguishable from those of aliphatic and benzylic acid methyl esters in such spectra. The ²⁹Si NMR of a silylated humic acid did not yield much information on the hydroxyl group functionality.