Iodine iSocyanate was added to polyisoprene and converted to the methyl iodocarbamate derivative and the iodourea derivative. The thermal stability of these modified polymers was compared to polyisoprene using simultaneously run thermogravimetric analysis and differential thermal analysis. Both modifications showed initial weight losses starting at about 125°C which correspond to cyclization reactions. Beyond 350°C, the iodourea derivative showed greater thermal stability than the polyisoprene or the methyl iodocarbamate derivative. This enhanced thermal stability is believed due to the formation of a semi-ladder polymer structure.