The self-esterification of two fulvic acid model compounds in methanolic solvents was studied by electrospray ionization mass spectrometry (ESI-MS). The strongly acidic tetrahydrofurantetracarboxylic acid rapidly self-esterified to form mono- and dimethyl esters when stored in methanol, even at reduced temperatures. The weakly acidic analogue, cyclopentanetetracarboxylic acid, reacted minimally under the same conditions. The use of 50:50 methanol/water as a solvent reduced self-esterification of the strong acid. However, the presence of water promoted the formation of multiply charged ions in the ESI mass spectra. The use of water and 50:50 acetonitrile/water as solvents eliminated self-esterification but the mass spectra still contained multiply charged ions. This study implies that the use of methanolic solvents with humic substances may compromise analytical data through the formation of methyl esters.