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Preparation of two-dimensional layered materials (2DLM) nanosheets is critical for both fundamental studies and applications. Here we present a systematic study to explore the fundamental factors to control the exfoliation of graphite and MoS2 in aqueous N-methylpyrrolidinone (NMP) using water as the cosolvent. Detailed materials characterizations suggested that the cosolvency significantly influences the yield and the stability of exfoliated nanosheets. The dependence of exfoliation on cosolvency was examined by fundamental factors including solid–liquid interfacial parameters, Hansen Solubility Parameter (HSP), and intermolecular-interaction-sensitive physical parameters of the water–NMP mixed solvent system. Fourier transform infrared spectroscopy (FTIR) study revealed that the water–NMP heteroassociated molecular structures, formed with the addition of water to NMP, could play an important role in the liquid exfoliation of layered materials. Our work provides a guide to rational design of a solvent sy...