Two-component sequential-interpenetrating polymer networks (IPNs) from castor oil-based polyurethanes and poly(methyl methacrylate) have been prepared. Component I-polyurethane was formed by reacting the hydroxyl functionality of castor oil with different diisocyanates in the stoichiometric NCO/OH ratio of 1.6. These polyurethanes were mixed individually with methyl methacrylate monomer and subsequently an interpenetrating network prepared by free radical polymerization initiated by benzoyl peroxide in the presence of crosslinking agent. The interpenetrating polymer networks, PU/PMMA IPNs, were moulded into tough and transparent films. The thermal stability of these IPNs was assessed by thermogravimetric analysis.