Melamine is widely used as a blowing agent in intumescent flame-retardant systems. However, when it was blended into the epoxy resin cured with hyperbranched (3-hydroxyphenyl) phosphate (HHPP), neither greater expansion nor improved limiting oxygen index (LOI) was obtained during combustion compared with that without melamine addition. When a portion of HHPP was replaced by polyethylene glycol (PEG), both the expansion degree and the LOI increased, along with the content of PEG, even if the same amount of melamine was blended. This phenomenon shows that the molecular chain structure of the cured epoxy resin based on HHPP has a substantial effect on the flame retardance. Only when the degraded products are sufficiently viscous, at the decomposition temperature of the blowing agent, can the material be bubbled to form an expanding charred crust. The morphologic structure of the crust of the epoxy resin cured with HHPP shows that there were many holes on the surface because the degraded products were too rigid to be bubbled, from which the gases formed by melamine evolved. With increasing PEG content, the degraded products became increasingly viscous and bulged more easily with the gases. No holes appeared on the charred crust of the sample cured with the mixture of HHPP and PEG at a ratio of 3:1. In this case, most of the gases were entrapped and caused the material to expand, resulting in the increases of expansion degree and LOI. (C) 2004 Wiley Periodicals.