Interpolymer complexes of poly (ethylene imine) (PEI) with an acrylic copolymer, and a polymer blend with identical structural units have been prepared. The stability constant [K], degree of linkage [0], and related thermodynamic parameters (e.g. Delta H-o and Delta S-o) of the two complexes have been found to be widely different. The discrepancy has been interpreted in terms of cooperativity, neighbouring group influence, and sequence combination of comonomer units in the copolymer chain.