A number of polyurethanes were synthesized by reacting castor oil with hexamethylene diisocyanate, varying the NCO/OH ratio. The polyurethanes were reacted with 2-hydroxyethyl methacrylate (HEMA) to prepare the interpenetrating polymer networks (IPNs) using benzoyl peroxide as the initiator and ethylene glycol dimethacrylate (EGDM) as the crosslinker. The IPNs are partly soluble in some of the solvents and are less resistant to alkali, but more resistant to acid. The solvent absorption is more pronounced in benzene than in toluene. A novel computerized LOTUS package was used to calculate the kinetic parameters. All the IPNs decomposed with 2-4% weight in the temperature range of 0 to 200 degrees C; 10% weight loss occurs at 300 degrees C and 40% weight loss occurs at 400 degrees C. There is a rapid weight loss from 10 to 90% in the temperature range of 400-500 degrees C. From the kinetic data, it is clear that the degradation process of the IPNs is slower in the temperature range 300-400 degrees C and faster in the temperature range of 440-560 degrees C.