Thermodegradation behaviors of novel aluminum phosphate/cyanate ester (AIPO(4)(KH550)/CE) composites were studied in detail. Results show that thermodegradation behaviors and kinetic parameters of AIPO(4)(KH550)/CE composites are greatly dependent on the AIPO(4)(KH550) loading. The addition of AIPO(4)(KH550) into CE resin changes the thermodegradation mechanism (mainly at the temperature lower than 450 degrees C) mid degradation process from two steps to three steps. Comparing with CE resin, AIPO(4)(KH550)/CE composites have lower initial degradation temperature and greatly higher char yield. Besides, for each thermodegradation step, the more the AIPO(4) content, the smaller the activation energy value is. All reasons leading to these outcomes are investigated intensively. (C) 2009 Wiley Periodicals, Inc. J Appl Polyrn Sci 113: 3427-3435, 2009