We demonstrate the synthesis of semiconductor Pb2-xSnxS2 nanocrystals with a cubic rock salt crystal structure in a composition range where this structure is unstable in the bulk. The cubic Pb2-xSnxS2 nanocrystals were prepared using a modified hot injection colloidal synthetic route. The x value is in the range 0.40 < x < 1. Even though these compositions lie in a region of the PbS-SnS phase diagram where no single phase exists, and despite the fact that PbSnS2 is a distorted orthorhombic phase, the Pb2-xSnxS2 nanocrystals are single phase solid solutions with cubic NaCl-type structure. Experimental evidence for this derives from powder X-ray diffraction (PXRD), electron diffraction, and pair distribution function (PDF) analysis. Elemental compositions determined using scanning transmission electron microscopy/energy dispersive spectroscopy (STEM/EDS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and electron energy loss spectroscopy (EELS) reveal a composition close to the nominal ones. The band gaps of the Pb2-xSnxS2 nanocrystals (0.52-0.57 eV) are blue-shifted by quantum confinement relative to that of the hypothetical cubic PbSnS2 phase which density functional theory (DFT) calculations show to be much narrower (0.2 eV) than in the case of orthorhombic PbSnS2 (1.1 eV). The Pb2-xSnxS2 nanocrystals exhibit a well-defined band gap in the near-IR region and are stable up to similar to 300 C above which they phase separate into cubic PbS and orthorhombic alpha-SnS.