Densities, excess volumes and heats of mixing, heat capacities, refractive indices and viscosities of various N‐methyl‐2‐pyrrolidinone (NMP)‐water mixtures are reported. The excess volumes and heat of mixing are negative and show their greatest deviations from ideality at 0.3 ‐ 04 mole fraction NMP. The viscosity of the system passes through a maximum in the same region of solvent composition. These data indicate strong interaction between NMP and water and suggest the existence of transient polymeric species of the type — (NMP · 2H2O)n̄.

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