Chain-extended polyurethanes (PUS) were prepared using castor oil and different diisocyanates such as toluene-2,4-diisocyanate and 4,4'-methylene bis(phenylisocyanate) as a crosslinker and different aromatic diamines like 4,4'-diaminodiphenyl methane and 4,4'-diaminodiphenyl sulphone as chain extenders. The effect of aromatic diamines on the swelling and thermal degradation behavior of PU have been discussed. A thermogravimetric analyzer (TGA) curve shows that all the chain-extended PUS are stable up to 194degreesC and that maximum weight loss occurs at 490degreesC. The TGA thermograms show that the thermal degradation of the PUS was found to proceed in two steps. The average molecular weight between crosslinks ((M) over bar (c)) was determined by swelling studies. The properties imparted by the aromatic chain extenders are explained on the basis of groups present in the diamines.