Five type II kerogens, shown by elemental analysis and Rock-Eval pyrolysis to represent a gradient of thermal maturity, were further characterized using a range of solid-state ¹³C NMR spectroscopic techniques. ¹³C cross polarization (CP) NMR spectra of the kerogens confirmed the well-established pattern of increasing aromaticity with increasing thermal maturity. Spin counting showed that CP observability was around 50% for the immature kerogens, and only 14–25% for the mature kerogens. Spin counting also showed that the direct polarization (DP) observabilities were >80% for all but one of the kerogens. Despite the large differences in observability between the two techniques, aromaticities derived from corresponding CP and DP spectra differed by only 1–15%. The RESTORE technique showed that the low CP observability of the immature kerogens was due mostly to rapid T₁ᵨH relaxation, whereas both rapid T₁ᵨH relaxation and slow polarization transfer contributed to the low CP observability of the mature kerogens.