The limits of isomorphous substitutions and the symmetry of synthetic binary solid solutions of paraffins C_nH_2n+2 with n=17-24 have been studied by X-ray diffraction and thermal X-ray diffraction. Principles of isomorphous substitutions of paraffin molecules are suggested which take into consideration the length (n), the symmetry (parity of n), and the intensity of thermal rotational motion of component molecules.