Solubility and freezing point data are presented for the system sulfur dioxide –N-methyl-2-pyrroIidinone (NMP). The enthalpy and entropy of solution are determined as −33.7 kJ mol−1and −94.6 J K−1 mol−1, respectively. The freezing point diagram shows maxima at, suggesting the presence of 2:1 and 1:1 NMP:SO2complexes. The existence of such complexes in the liquid state is supported by viscosity and permittivity data. Nuclear magnetic resonance and Raman spectroscopic data suggest an overall shift of electron density from the nitrogen and the carbonyl carbon of NMP to the O of SO2, with weakening of the CO bond of NMP.