A modified single fiber fragmentation test (SFFT) procedure, that permits separation of the fiber break and fiber/matrix (F/M) debond propagation events, was employed to characterize the (F/M) interface toughness of dry and water saturated E-glass/vinylester. By focusing solely on the debond propagation event, and by measuring the critical load for debond propagation, fracture mechanic analysis enabled determination of the fracture toughness of the fiber/matrix interface. After immersion in seawater, the interface was substantially degraded. The fracture toughness was reduced by approximately a factor of two.