The presumptions are given on which the thermodynamics of eutectoid oligomer- and copolymer systems is based. It is typical for both systems that crystallization segregation of non-crystallizable chain units occurs on “molecular scales”. Solid solution is therefore possible only by forming crystal lamellae with diffuse basal defect layers. The existence of these inhomogeneous micro-phases is demonstrated. The solid solution is limited to a maximum number of components for thermodynamic reasons. The understanding of isobaric state diagrams and of melting in eutectoid oligomers and copolymers with the aid of thermodynamics includes that chain and colloid structure are related in a fundamental manner. That this holds true in eutectoid oligomers and copolymers is verified by a combined description of caloric melting curves and WAXS- and SAXS-measurements. Some fundamental aspects and problems are discussed.