The ABC spectrum of the vinyl protons of methyl acrylate is analysed in detail. There are two exact solutions of both the 60 and 100 Mc./sec. spectra, which give identical transition energies and assignments, even though the molecular parameters obtained differ considerably. In the 100 Mc./sec. spectrum, the two solutions obtained have coupling constants of different relative signs but cannot be differentiated by tickling experiments, in contrast to the weakly coupled case. The reasons for this and the general problem of indeterminacy in the analysis of n.m.r. spectra are discussed.