The compositions of humic acids (HAs) from various Histosols in North America and Europe, of similarly treated plant-extracted materials (PEMs), of coal-extracted humic acids, and of International Humic Substances Society (IHSS) Florida peat were quantified by solid-state 13<\sup>C nuclear magnetic resonance (NMR). In order to obtain quantitative intensities, the peak areas in direct-polarization 13-kHz magic-angle spinning (DPMAS) 13C NMR spectra were corrected for incomplete relaxation by factors measured in cross-polarization spin-lattice relaxation time (CP/T1) experiments with total sideband suppression (TOSS). The elemental compositions (%C, %H, %O + N) of a peat sample, 8 HAs and 2 PEMs were estimated from the NMR results and compared with chemical analyses, as well as solution NMR for two of the HAs. The results are in good agreement, which shows that DPMAS corrected by CP/T1–TOSS permits quantitative characterization of HAs and PEMs. The compositions of the PEMs deviate significantly from those of the Histosol HAs. The compositions in terms of nine types of chemical groups were computed. The investigated HAs consist of more than 60% of aromatic and CO carbons (including both carbonyl and carboxyl groups). Previous cross-polarization magic-angle spinning (CPMAS) NMR experiments have significantly underestimated the ratio of sp2– to sp3–carbons; in particular, the true COO carbon fraction is a factor of two larger than estimated by CPMAS NMR. In spite of their wide range of geographical origins, the compositions of the Histosol HAs appear to be relatively uniform, suggesting that the search for a general model of Histosol HA structure is worthwhile. Eight models proposed in the literature do not reproduce the experimentally determined compositions, but a few models show promising partial agreement.