The phase behaviour of blends of chlorinated polyethylene, polyvinyl chloride (PVC) and chlorinated PVC with random copolymers of caprolactone and caprolactam has been investigated and the results correlated with a binary interaction model. The known miscibility of polycaprolactone in the chlorinated polymers is not compromised until a relatively high lactam content in the copolymer is attained. The incorporation of segmental interaction parameters, derived from separate studies involving polyamides, polyesters and their copolymers, has provided an estimate of the unknown parameters. Using only six parameters to account for experimental data has provided moderate success; however, it is clear that the representation of the chlorinated polymers, as pure copolymers of methylene (-CH2-) and chlorine containing (-CHCl-) segments, is not sufficient to describe those blends. The reasons for this have been discussed in terms of the influence of configuration of species upon segmental interactions. Exploratory calculations are also presented depicting highly unfavourable interactions between aliphatic polyamides and chlorinated polymers and to describe how the window of miscibility in blends of the latter with semi-aromatic polyesters is affected by the methylene content of the polyester.