The influence of the configuration of elementary chemical species, such as methylene, amide and phenyl groups, upon the phase behaviour of polyamide blends has been examined with reference to poly(hexamethylene isophthalamide), (nylon-6I) and poly(m-xylene adipamide) (MXD6), respectively, in aliphatic polyamides. The observable differences have been correlated with a binary interaction model to illustrate how the segmental interaction parameters are affected. It is proposed that the small differences detected may be attributed to the influence of neighbouring species on the interactions and that segments should be considered in terms of their triad groups. This is analogous to the approach already applied to blends of random addition copolymers. Additional experimental investigations, accompanied by predictive calculations, are also presented for blends of MXD6 with various aromatic polyamides, including the aramid poly(m-phenylene isophthalamide), and various aliphatic copoly(ester-amides).