C-13 CP/MAS NMR techniques have been used to evaluate the structure of four hyper-cross-linked polystyrene resins. Single-pulse excitation (SPE) C-13 spectra have allowed the various types of carbon atoms present in the resins to be quantified, and similarly H-1 CRAMPS spectra have facilitated quantification of the various hydrogen atom types. Two hyper-cross-linked resins prepared by exhaustive methylene bridging of conventional lightly cross-linked styrene-divinylbenzene resins have very high quaternary aromatic carbon contents. These can be explained adequately only if vert high levels of double methylene bridging of aromatic groups occurs. Approximately 10% of aromatic groups also retain a chloromethyl group from incomplete methylene bridging. Two further hyper-cross-linked resins reportedly prepared from conventional heavily cross-linked divinylbenzene (55% technical grade) resins appear in one instance to exploit residual unreacted double bonds (and possibly additional divinylbenzene) to generate the secondary cross-links, and in another to use SOCl2/Lewis acid to introduce very high levels of sulfoxide secondary cross-links. Dynamic NMR experiments suggest that the latter two hyper-cross-linked species retain more flexibility and mobility than those prepared via more conventional methylene bridging techniques.