The effect of isothermal aging on interlaminar shear strengths of quasi-isotropic composites laminates were evaluated by using ASTM D 3846-94, Standard Test Method for In-Plane Shear Strength of Reinforced Plastics. Thermoset bismaleimide (IM7/5260) and thermoplastic polyimide (IM7/K3B) were investigated to determine the aging degradation in different matrix materials. The laminates were 24 plies and had a lay up of [-45/90/45/0]3S. Aging temperatures were 300, 350, and 400°F and aging time varied from a few hours to 28 months. Nonaged specimens were also tested for baseline values. Significant differences were noted between these two different matrices. Isothermal aging for IM7/5260 initially caused posteuring effects prior to an overall decrease in interlaminar shear strength. Strength degradation was directly dependent on both aging temperatures and times. In addition, accelerated aging effects at 400°F were observed. For the IM7/K3B specimens no postcure effects were observed. The interlaminar shear strength decreased for longer aging times, but showed little variation for different temperatures. Aging at 400°F did not show any accelerated aging effects. The interlaminar shear strength results were compared with previously obtained compression after impact data. Similar trends in strength degradation were observed.