The experimental systems considered in this paper are isobaric binary mixed crystals, and their properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. More in particular, the excess behavior is examined for families of systems where the components of each of the member systems belong to a group of chemically coherent substances. The accent is on a group of 18 systems composed of n-alkanes in the range from C11H24 to C26H54, such that the components of an individual system are either neighbors or next nearest neighbors. In these systems mixed crystalline solid forms are stable in which the molecules have rotational freedom around their long axis. One of these forms is rotator I, and it is this form for which thermodynamic mixing properties have been determined. The magnitudes of the excess enthalpy and excess entropy are system-dependent and can be correlated to the geometric mismatch, i.e., the relative difference in number of carbon atoms of the molecules of the components of the system. The quotient of the two excess properties, on the other hand, is shown to be system-independent: it corresponds to a temperature which is characteristic of the family as a whole. For the rotator n-alkane family, its value is 320 K. The result obtained for the n-alkanes gives support to the treatment (of a similar nature) we applied earlier to the common-ion alkali halide systems and the p-dihalobenzenes. We propose to refer to such groups of systems of mixed crystals as a "perfect family of mixed crystals."