The objectives of this paper were to investigate the water absorption and hygrothermal aging behavior of rubber-toughened poly(butylenes terephthalate) (RTPBT) with and without short glass fiber (SGF) reinforcement. The rubbers used in the study were AX8900 and EXL2314, both of which are acrylate-based terpolymer. The effect of the hygrothermal aging on its fracture properties was also studied. The kinetics of the water absorption study were carried out on the injection-molded samples of the RT-PBTs and the SGF-reinforced rubber-toughened PBT (SGF-RTPBT) at three immersion temperatures, 30, 60 and 90degreesC, for a total of 450 h. The study of the deterioration caused by the hygrothermal aging was conducted by investigating the fracture parameters and flexural properties of all the materials as both hygrothermally aged (HA) and redried state (RD). The modes of the failure of HA and RD samples were studied using the scanning electron microscopy (SEM) technique. It was found that all the samples conformed to Fickian behavior and the kinetics of absorption exhibited a strong dependency on the rubber types, presence of SGF, as well as the immersion temperature. Generally, SGF-RT-PBT showed a better resistance to hygrothermal aging than that of RT-PBT and PBT, though a declining trend was observed in the fracture parameters, K, and G,. However, an opposite observation was exhibited in the flexural properties in some, but not all cases. Finally, the results obtained from SEM micrographs showed that permanent damage occurred in the materials and the hygrothermal aging had suppressed the plastic deformation ability of the PBT matrix and both types of impact modifiers where brittle failure was observed. Fiber pull-out was apparently the failure mode of the SGF-reinforced materials. (C) 2004 Wiley Periodicals, Inc.