The experimental temperature versus composition phase diagram of the systemn-C8H18‒n-C10H22obtained 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 C10. The calculated phase diagram, obtained by thermodynamic analysis, is in full agreement with the experimental one. The value of the coefficient of crystalline isomorphism, ϵimfor this system is 0.76.