The swelling in common (benzene) and selective (CCl4) solvents and small angle X-ray scattering (SAXS) of interpenetrating polymer networks (IPNs) of PPO/PMMA in all composition intervals are studied. It is shown that in both solvents IPNs swell better than the pure crosslinked polymers; that may be caused by a smaller degree of crosslinking of each component in IPN in comparison with the crosslinking of pure polymers. This leads to lower network density in IPNs than in the networks of pure polymers. The unusual effect is found: the IPN enriched with PMMA has the maximum swelling in CCl4, whereas PMMA itself does not swell in this solvent. SAXS measurements show that ordering in IPNs, i.e. microphase separation (MPS), is absent, but there are structural heterogeneities in the initial dry state of IPNs which are typical for these systems. These heterogeneities grow after swelling in a selective solvent. We suggest that PMMA units cluster together in poor solvent and as a result the internal stresses arise during swelling, which causes the large swelling of IPNs in selective solvent. the structure of the asymmetrical IPNs does not change with temperatures increasing up to 95 degrees C and dropping down to room temperature; as a consequence, the equilibrium swelling is also insensitive to the temperature. On the contrary, the structure of symmetrical IPNs changes with heating and comes back very slowly by returning to room temperature. (C) 2000 Elsevier Science Ltd. All rights reserved.