The environmental stress cracking (ESC) behavior of three carbon fiber-reinforced high-performance thermoplastic composite systems and a thermoplastic toughened thermoset composite system, in the presence of organic solvents, was investigated. Three-point bending/ flexure tests were conducted on [90°]4s (unidirectional) composite coupons. Results from the flexure tests showed a 10 to 30% drop in the bending strength of the composite specimens when solvents were applied just prior to the testing. Scanning electron micrographs of the fracture surfaces show differences in the failure modes as well as the microstructure. ESC was observed in the cross-ply specimens after solvent exposures from an hour up to a day, in the absence of external mechanical loads due to residual stresses. The effect of different solvents on the polymers and the resulting ESC phenomena could partially be explained by the differences in the solubility parameters of the concerned solvents and polymers.