AbstractAn investigation was conducted to evaluate the validity of various test methods in the dtermination of in‐plane shear moduli of semicrystalline, poly(phenylene sulfide) (PPS) thermoplastic composites. Materials studied included composites based on an orginal, non‐optimized composite grade of PPS reinforced with carbon and glass fibers, and another based on the new improved, high‐ductility, rapid crystallizing, composite grade of PPS along with optimally sized glass fiber reinforcement. Specimens were prepared in a matched die mold, either as‐molded or annealed. It was found that shear test methods, such as ±45° in plane, Iosipescu, and 45° pff‐axis tension, are validfor shear modulus measurements of PPS composites if the material has no internal stress cracks. For example, there was no significant difference for in‐plane shear moduli determined from these three shear testsw for the improved, thoughened PPS composites. This was attributed to the thoughness and fast crystallization of the PPS composites. This was attributed to the thoughness and fast crystallization of the PPS matrix resin, and the strong fiber/maxtrix interface, however, the ±45° in‐place shear test should be used with caution for certain thermoplastic composites,. Becuase of the ±45° lay‐up, the annealed[±45]2soriginal PPS composite la, omates siffered jogj resodia; stresses and were observed to have stress‐related cracks. Consequently, the measured shear moduli are lower than the true values. Since we have demostrated that some shear tests amy not be appropriate for modulus measurements of certain composites, it is important to choose the proper test method for the shear characterization of