The experimental temperature versus composition phase diagram of the system n-C8H18-n-C10H22 obtained by thermal analysis and X-ray powder diffraction is characterized by a simple eutectic and little miscibility in the solid state. The eutectic invariant is at 211.7 K and the eutectic composition is at a molar composition of 0.16 in C-10. The calculated phase diagram, obtained by thermodynamic analysis, is in full agreement with the experimental one. The value of the coefficient of crystalline isomorphism, epsilon(m)(i), for this system is 0.76.