Chitosan is a natural carbohydrate biopolymer derived by deacetylation of chitin. Reaction of chitosan with carbonyl compounds leading to formation of Schiff bases have been reported in the older literatures and their importance has not yet fully realized. The present work involves modification of chitosan by benzophenone, acetophenone, cinnamic acid or phenylacetic acid. The thermal stabilities of the prepared polymers were compared with chitosan polymer. Thermal degradation of the modified polymers has been studied in order to determine the nature of the degradation products and the activation energies of the degradation were calculated.