Synthetic solid solutions of n-paraffins C23H48and C21h44 have been studied by the thermoroentgenographic method. It is shown that the limits of isomorphous substitutions of molecules in these paraffins with molecular ratios C23: C21 = 1:2, 1:1 and 2:1 are temperature dependent. At rising temperatures all solid solutions first break down to form two phases which coexist in a narrow temperature range (3 –5°C) and then become homogeneous at further elevation of temperature. Isomorphism in the system is established to be violated in the course of intensive structural rearrangements; it gets restored shortly (7 – 9° C) before the orthorhombic-hexagonal polymorphic transformation. A decrease in the limits of isomorphous substitutions which occurs at rising temperatures is explained by differences in energy states of paraffin chains having different lengths: relatively long chains oscillate about their own axes whereas shorter chains perform complete rotations and get “separated” from solid solutions.