Polysulphones with cross-linkable pendant vinylbenzyl groups (PSF-VB) were prepared via chloromethylation of commercial polysulphones. The curing reactivity of PSF-VB was investigated by di †erential scanning calorimetry. Interpenetrating polymer networks (IPNs) were prepared based on bisphenol A diglycidyl ether (DGEBA) and PSF-VB, where DGEBA was cured by 4,4@-diaminodiphenyl sulphone and VB groups of PSF-VB were radically polymerized using dicumyl peroxide (DCP). Polysulphones having pendant benzyl groups (PSF-Bz) were also prepared and used as non-reactive modiÐers. The fracture toughness for the resulting epoxy/PSF-VB IPN increased by 65% (K IC ) with no loss of mechanical properties on 10 wt% addition of PSF-VB (7É9 mol% VB unit, MW 74 000). Non-reactive PSF-Bz was less e †ective than PSF-VB. Although the PSF-Bz modiÐed resin had a particulate structure, the morphologies of the PSF-VB/epoxy IPNs were not clear from scanning electron micrographs. Furthermore, the epoxy/PSF-VB IPNs had higher solvent resistance than the epoxy/PSF-Bz blends. Morphological behaviour, modiÐcation results and high solvent resistance of the cured epoxy/PSF-VB resins indicate that crosslinked PSF-VB and the epoxy network entangled fully in the presence of DCP.