Solid state n.m.r. (wide-line n.m.r.) has been used to investigate the dynamic structures in the solid state of a series of π–π molecular complexes of the acceptor molecules tetracyanoethylene, pyromellitic dianhydride, tetracyanobenzene and hexafluorobenzene with the donor molecule pyrene. The nature of the motions has been deduced, and the results are discussed in terms of the crystal structures found by X-ray diffraction measurements.