A number of semi-interpenetrating polymer networks (semi-IPNs) were synthesized by reacting polyurethanes (prepared from castor oil and various diisocyanates) and a phenolic resin (prepared from cardanol and furfural). The physico-chemical properties of the semi-IPNs have been investigated. The thermogravimetric analysis of polymers was followed using a computer analysis method for assigning the kinetic mechanism. Various kinetic equations have been used to evaluate the kinetic parameters. The suggested mechanism of degradation of the semi-IPNs is based on the kinetic parameters.