The internal rotation of five kinds of N,N-dimethylformamide (DMF) derivatives was studied by dynamic nuclear magnetic resonance (n.m.r.) and Raman spectroscopic measurements in relation to the dissolution mechanism of polyacrylonitrile (PAN). The dissolution temperature (T-sol) of PAN was determined by the steric effect of the solvent molecules, i.e. the higher the bulkiness of a substituent, then the higher was the T-sol of PAN. The intermolecular interaction between the nitrile group and the solvents was evaluated by Raman spectra in terms of the large carbonyl shift (nu(c=o)). Dynamic H-1 n.m.r. spectroscopy measurements revealed that the value of the coalescence temperature (T-c) was not proportional to the steric factor, suggesting that the solvent properties are not determined solely by the latter and that internal rotation of the solvent molecules is defined by the extent of looseness in the packing of the solvent molecules in the liquid state. The significance of both intra- and intermolecular factors of the solvent molecules in the dissolution of PAN was also discussed.