Abstract Castor oil was transesterified with pentaerythritol thereby introducing reactive hydroxyl groups. The method of matrix‐assisted laser desorption/ionization time‐of‐flight mass spectroscopy was used to determine the components of the castor oil pentaerythritol alcoholysis products. Simultaneous interpenetrating polymer networks of pentaerythritol modified castor oil and diphenyl‐methane 4‐4′ diisocyanate polyurethane (P1‐CO‐PU) and polystyrene (PS) were synthesized using benzoyl peroxide as the initiator and divinyl benzene as the crosslinker. The effect of PU/PS composition on the morphology, miscibility and physical properties of the resulting IPNs was investigated by scanning electron microscopy, dynamic mechanical thermal analysis, measurements of mechanical properties and resistance to chemical reagents. The patterns of the extent of the phase separation and the characteristics of the interpenetration reached as a function of the components were deduced.