Sequential interpenetrating polymer networks (IPNs) composed of poly(butyl acrylate) as the first network and modified polystyrene containing proton-donor hydroxyl groups [PS(OH)] as the second network are studied by d.s.c., transmission electron microscopy and dynamic mechanical analysis. The apparent miscibility between the components increases considerably as the hydroxyl content in PS(OH) increases. However, differing from the corresponding blends of the linear polymers, for which similar to 2 mol% hydroxyl introduced into PS(OH) leads to complete miscibility, the IPNs always show two-phase structures even when the hydroxyl content in PS(OH) is as high as 30 mol%.