We examined particle size fractions from the surface horizons of a native untilled grassland site and an adjacent field cultivated for 65 years in the Peace River region of British Columbia, Canada. While cultivation resulted in the loss of approximately 50% of the C and N in the soil, there was little change in the distribution of particle sizes, in their C/N ratios, and in the fraction of C found as carbohydrate. Changes in the nature of the organic C were generally small, but obvious differences were seen in the 13C CPMAS NMR spectra of two of the size fractions. Compared with the native grassland, the sand fraction from the cultivated field had a lower proportion of 0-alkyl C and a loss of resolution. The clay fraction from the cultivated field had lower 0-alkyl and higher alkyl C than its grassland counterpart. Subtraction of linear combinations of spectra of the whole soils from the two sites showed a 10% increase with cultivation in the proportion of humified to decomposable organic C. All of these trends are consistent with a greater degree of decomposition in the cultivated site. The changes are small and do not suggest a serious degradation in the quality of the soil organic matter, compared with the large change in quantity. We also examined factors that might affect the quality and quantitative reliability of the 13C CPMAS NMR spectra. Dithionite treatment was not very effective in improving resolution or fraction of observable C for these samples. The latter was in the range of 10–30%, largely a result of the association of organic matter with paramagnetic iron. For this reason, comparison of relative areas must be interpreted with caution and confined to samples with reasonably similar contents of C and iron.