The effect of crosslink density in interpenetrating polymer networks (IPNs) with controllable hydrogen bonding composed of poly(butyl acrylate) (PBA) and polystyrene modified with hydroxyl-containing units [PS(OH)] on miscibility and morphology is studied. The results from d.s.c. and transmission electron microscopy (TEM) show that the crosslink density of PBA displays a two-fold effect on miscibility of the IPNs. For the IPNs made from immiscible polymer pairs with low hydrogen bonding density, crosslinking improves the degree of mixing, while for the IPNs composed of miscible polymer pairs due to hydrogen bonding, crosslinking causes microheterogeneity. An apparent discrepancy between the results of dynamic mechanical analysis and d.s.c. or TEM is observed and explained.