The demand for biodegradable polymers produced from renewable natural resources continues to grow as environmental concerns increase. Biodegradable plastics derived from agricultural feedstock are a new generation of materials capable of reducing the environmental impact in terms of energy consumption and greenhouse effect in specific applications to perform as traditional/conventional plastics when in use and are completely biodegradable within a composting cycle through the action of living/micro-organisms. The objective of this study is to examine the potentiality and performance pattern of Soy protein isolate (SPI) resin, modified with various concentrations of thiosemicarbazide (TSC), as a thermoplastic to substitute some conventional petroleum-based plastics. The spectral, thermal, morphological properties and the biodegradability of the modified resin have been investigated. The spectral studies indicate that TSC is not crosslinked with the protein moiety; rather, it acts as a modifier. Thermogravimetric analysis of the modified material has been followed using a computer analysis method (LOTUS package) developed by us for assigning the degradation mechanism. A number of equations have been used to evaluate the kinetic parameters. The degradation mechanism has been ascertained on the basis of the kinetic parameters. It is expected that, this environment-friendly, fully biodegradable and sustainable TSC-modified SPI green plastic could be commercially used for making molded products. (c) 2006 Wiley Periodicals, Inc.