This paper studies complexation behaviour between block ionomers with metal sulfonate groups and pyridine-group-containing copolymers, i.e. poly(styrene-co-4-vinyl pyridine) (STVP) and poly(methyl methacrylate-co-4-vinyl pyridine) (MVP) in both solution and bulk. The block ionomers based on poly(styrene-b-ethylene-co-butene-b-styrene) (SEES) with metal sulfonate groups were prepared by sulfonation of SEBS followed by neutralization. In the blend solutions in tetrahydrofuran, intercomponent complexation is evidenced by an unusual increase in the reduced viscosity relative to the solutions of SEBS and the copolymers. This complexation was observed by dynamic light scattering (DLS) measurements to take place at concentrations as low as 0.01 g dl(-1). DLS also shows the coexistence of two kinds of complex differing considerably in size, which are formed from STVP with single block ionomers and block ionomer aggregates, respectively. Among the different metal salts of alkali, alkaline earth and transition metals studied, copper and nickel show the greatest capability of complexation. In the solvent-cast blends of ionomer/MVP and ionomer/STVP, T-g is much higher than expected for the usual miscible blends, and varies with the blend composition in a way similar to the reduced viscosity of the blend solutions, with a similarly located maximum; this implies that the apparent increase in T-g of the blends is caused by complexation which occurs at dilute solution and is preserved in the casting process.