The phase diagram of the binary n-alkane system C18H38-C19H40 is determined by calorimetric and X-ray diffraction methods. The present experimental results and previous work on pure C,,H,, and C,,H,, paraffins allow us to identify all the phases of the binary system. The phase diagram exhibits no less than six distinct solid domains : three are one-phase regions, triclinic (T), orthorhombic (O) and face-centered orthorhombic, called rotator phase (RI), and three are two-phase equilibria [T + O], [T + RI] and [O + RI]. The two regions [O] and [RI] occur on large composition ranges (from the pure constituent C19H40). The solid-liquid equilibria are separated by a eutectic invariant. They can be explained in terms of crossed isodimorphism.