The solvent properties of the binary solvent carbon disulfide/N-methylpyrrolidone (NMP) have been investigated using density, viscosity, solvatochromic, and FTIR measurements. The excess molar volumes were negative throughout the entire range of CS2/NMP ratios suggesting complex formation or preferred solvent structures at intermediate ratios of the solvents. The viscosity measurements suggested either complex formation between CS2 and NMP or very complex self-association equilibria. FTIR data did not support the idea of specific interactions between CS2 and NMP, but did show that complex changes in self-association were occurring. The solvatochromic studies indicated that at approximately a 1:1 volume ratio, the dipolar state of the binary solvents was greater than for either solvent separately. All the information together suggested that some type of NMP chain oligomer may be an important factor in understanding the synergistic extraction of coal in CS2/NMP solvent.