A thermodynamic treatment of eutectoid crystallization in oligomer mulit-component systems comprised of homologues allows isobaric state diagrams to be predicted. This description relies on the definition of an inhomogeneous extended-chain micro-phase. The thickness distribution of crystals and the chain-length distribution are strictly interrelated because chain ends are squeezed into defect layers. The clear possibilities of a quantitative thermal analysis of melting in eutectoid oligomer mixtures are demonstrated. Next we consider crystallizable stereo-regular sequences in copolymers as thermodynamic components. If the co-units are irregularily distributed, these copolymers display eutectoid crystallization with a unique correlation between chain structure and the thickness distribution of the mixed, extended-sequence micro-phases. The segregation demanded by the thermodynamics seems to take place even in semi-crystalline networks where the chains are linked as components in the network. It is then possible to describe the kinetics of thermally and strain-induced crystallization. Here, one has to combine the thermodynamics of eutectoid copolymers with the van der Waals model of real networks. Measurements with a micro-stretch calorimeter extend substantially the analytical power of caloric measurements. In combination with X-ray investigations, a relatively consistent description of the cluster structure in semi-crystalline copolymers and networks is obtained. Both eutectoid oligomer multi-component systems and eutectoid copolymers come relatively near to thermodynamic equilibrium. This can only be explained if the segregation of components takes place on a very local scale within thermodynamically equivalent sub-systems of finite size. These processes are strictly synchronized by the thermodynamics so as to allow them to be studied by macroscopic measurements.