(Solid + liquid) phase diagrams have been determined for (N -methyl-2-pyrrolidinone + benzene, or toluene, or 1,3,5-trimethylbenzene, or ethylbenzene, or chlorobenzene, or 1,2-dichlorobenzene, or 1,2,4-trichlorobenzene, or 1,1,1-trichloroethane, or dichloromethane). Solid addition compounds form with the empirical formulae: { (C 5 H 9 NO) 2 · C 6 H 6 }, (C 5 H 9 NO · 2ClC 6 H 5 ),(C 5 H 6 NO · Cl 2 C 6 H 4 ) ,(2C 5 H 9 NO · Cl 3 C 6 H 3 ) . All are congruently melting compounds. Compound formation is attributed to a charge-transfer interaction with the benzene, or chlorobenzene, or 1,2-dichlorobenzene, or 1,2,4-trichlorobenzene acting as electron acceptors and the nitrogen, or oxygen in N -methyl-2-pyrrolidinone acting as electron donors.