Co-microencapsulated ammonium polyphosphate and dipentaerythritol [M(A&D)] was prepared using a melamine-formaldehyde (MF) resin by in situ polymerization method, and characterized by XPS. The co-microencapsulation of ammonium polyphosphate and dipentaerythritol (DPER) leads to a great improvement in water solubility of the additives. The flame retardant effect of M(A&D) in polypropylene (PP) is evaluated using limiting oxygen index (LOI) and UL 94 test, and the water resistance of the PP/M(A&D) composites is also studied. The flame retardant properties and water resistance of the PP/M(A&D) composites are much better than the ones of the PP/APP/DPER composites. Moreover, the thermal stability of the PP/M(A&D) composites is improved compared with the PP/APP/DPER composites. POLYM. ENG. SCI., 48:2426-2431, 2008. (C) 2008 Society of Plastics Engineers