High-yield (9.4 T) Fourier transform ion cyclotron resonance (FT-ICR)mass spectra of standard Suwannee River humic and fulvic acids have been obtained by use of laser desorption (LDI) and electrospray (ESI) ionization. The LDI FT-ICR mass spectrum was similar to those observed previously, with ions at essentially every nominal value, 200 less than or equal to m/z less than or equal to 800. In contrast, the ESI FT-ICR mass spectrum, although still containing ions at most values in the 200 less than or equal to m/z less than or equal to 800 range, was dominated by a relatively few prominent species. ESI FT-ICR mass spectra of standard humic and fulvic acid isolates were similar. Although many ionic species appeared in both fulvic acid and humic acid ESI FT-ICR mass spectra, the fulvic acid mass spectrum contained more highly charged species. Subfractions of the fulvic acid mixture isolated by an HPLC procedure yielded similar mass spectra. The stability of high-mass ions produced by ESI combined with the high-mass resolution capability of FT-ICR MS allow for precise determination of molecular masses, from which molecular formulas may be obtained by mass alone. Future two-dimensional FT-ICR MS2 determinations of humic and fulvic acid structures should be feasible by use of collisionally induced and multiple-photon dissociation techniques.