The compression after impact (CAI) properties of bismaleimide (BMI) matrix composites manufactured by resin transfer molding (RTM) were significantly improved by ex-situ RTM technique. The thermoplastic polyetherketone with a functional group of phenolphthalein (PAEK) was used as toughener. The optical microscopy images of the cross-section of post-impact specimens revealed that the delamination resistance of specimens toughened through ex-situ RTM technique was dramatically improved. The energy absorption mechanism of composites toughened through ex-situ RTM technique was changed from the delamination to fiber fracture, which contributed to the improvement in CAI. The particle microstructure in interlaminar region of composites toughened through ex-situ RTM technique revealed that a reaction-induced phase decomposition and inversion happened in the interlaminar region. The BMI particles were surrounded with the PAEK phase, which can significantly improve the delamination resistance of composites. The in-plane static mechanical properties of G827/BMI composite toughened through ex-situ RTM technique were very well kept. (C) 2008 Wiley Periodicals, Inc.