After reviewing some recent studies on the characterisation of coke deposits on fluid catalytic cracking (FCC) and hydroprocessing catalysts by solid state C-13-MMR, the quantitative structural information that has been obtained through the use of demineralisation of FCC catalysts to provide coke concentrates for analysis will be described. The deactivated catalysts investigated contain only approx. 1% (w/w) carbon and were obtained both from refinery units operating with heavy feeds and from laboratory fluidised-bed tests with n-hexadecane. As for other carbonaceous materials, the use of a low-field field strength in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled most of the carbon to be detected and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. Although stripping the catalysts gives rise to highly aromatic cokes (aromaticity>0.95), even for n-hexadecane, differences in feedstock composition are still reflected in the structure of the resultant cokes with those derived from n-hexadecane containing less condensed aromatic nuclei than those from heavy feeds.