In this research, a new synergistic mechanism based on an acid-buffer action for cyclotriphosphazene (CPZ)/melamine cyanurate (MCA) flame retardant epoxy resin (EP) was proposed. This mechanism broke through the conventional well-recognized phosphorus-nitrogen interaction one. It revealed that CPZ had not only acid-catalytic charring but also acid-catalytic degrading effect on EP. The former that occurs in higher temperature range to improve the flame resistance in the condensed phase is a mechanism generally accepted for the phosphorus flame retardant, but the later that occurs in lower temperature range to deteriorate the flame retardance is usually ignored by the people. For CPZ/MCA flame retardant EP, the produced organic base from decomposed MCA can neutralize the acids from CPZ. Decline of the acidity effectively weakened the acid-catalytic effect, and reduced the volatiles release rate of the degraded resin in the initial stage, thus slowing down the combustion in the gaseous phase. With increasing temperature, the neutralized products were converted to the phosphorus-containing acids again to promote the formation of the chars. A series of characterizations such as vertical burning test, X-ray photoelectron spectra, micro-scale combustion calorimetry, thermogravimetric, and differential thermogravimetric analysis of the flame retardant materials and the pH value detection of the corresponding carbonation products were performed to investigate the acid-buffer mechanism. The experimental results including no NP forms in the condensed phase obviously improved flame retardance and increased degradation temperature of CPZ/MCA/EP compared with CPZ/EP, as well as the enhanced pH value of the former carbonation residue confirmed the above mechanism. POLYM. ENG. SCI., 55:1046-1051, 2015. (c) 2014 Society of Plastics Engineers