Broad-line n.m.r. investigations of a series of complexes of boron trifluoride with amine donor molecules (e.g., BF3· NH3) reveal the presence of molecular motion in the solid state. From the temperature dependence of the linewidths, activation energies are derived for the reorientation process. The observed values of the second moments of the absorption signals are compared with theoretical values calculated from the known X-ray structures assuming motion of different parts of the complexes. The motions are discussed in relation to the crystal and molecular structures of the various complexes.