Coke has been concentrated from two deactivated FCC refinery catalysts via demineralisation to facilitate detailed characterisation by solid state C-13 NMR, The catalysts were obtained from runs with a residue feed (5% Conradson carbon) and a hydrotreated vacuum gas oil (HVGO). As for solid fuels, the use of a low-field spectrometer in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled quantitative carbon skeletal parameters to be obtained for the cokes, Internal standard measurements demonstrated that most of the carbon was observed by SPE and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes, SPE gave much higher values for both the carbon aromaticities and the proportions of non-protonated aromatic carbon than the less quantitatively reliable cross-polarisation (CP) technique. Differences in feedstock composition were reflected in the structure of the cokes with the aromatic nuclei being more highly condensed in the residue-derived coke and corresponding to 15-20 pericondensed aromatic rings.