Demixing is a well-known phenomenon present in multi-component liquids and polymer blends, conventionally modeled by the Flory-Huggins free energy approach. In liquid crystals, it can result in a wide coexistence range of nematic and isotropic phases [1]. Determination of the respective temperature-concentration phase diagram is often a rather tedious task, due to many varying parameters: the relative volumes of the phases, the nematic order parameters of the individual mesogens in the nematic phase, as well as the concentrations of components in the nematic and isotropic phases. We demonstrate that deuteron NMR is a particularly useful experimental tool for the study of molecular segregation in nematics, providing for a simultaneous measurement of all the above parameters, both in bulk and confined systems, e.g. in liquid crystalline microemulsions, in doped liquid single-crystal elastomers, and in photoisomerizable nematics.