Polyurethane-phosphate combination is a novel intumescent flame retardant system. However, because of the water solubility and migration of phosphate, the intumescent system is not permanent. This problem might be solved by the technique of microencapsulation. Two types of microcapsules of di-ammonium hydrogen phosphate (DAHP) with, respectively, gelatin-polyurea shell and poly(vinyl alcohol)-polyurethane shell were synthesized. Chemical and physical structures of the DAHP microcapsules were characterized. Thermal degradation of the DAHP microcapsules exhibits characteristics of an intumescent system. Thus, it is expected that the microcapsules might be useful for improving the reaction to fire of various polymers. (C) 2005 Elsevier B.V. All rights reserved.