A fracture mechanics model was developed for determining the fracture toughness of the fiber/matrix (F/M) interface based on a modified test procedure for the single fiber fragmentation test (SFFT). After loading the specimen until the first fiber fracture and instantaneous debonding events occur, the specimen is unloaded and loaded again until the debond propagates. The critical load for debond propagation is measured and is used with a fracture mechanics analysis to determine the interface fracture toughness. The analysis considers also friction between the fiber and matrix in the debonded region. To obtain the necessary data for calculation of residual radial stress at the F/M interface due to matrix cure shrinkage, simultaneous measurements of dynamic modulus and cure shrinkage were conducted on the matrix (vinylester) during cure. Tests employing E-glass/vinylester SFFT specimens provided fracture toughness values of G cd = 62 J/m 2 (frictionless) and 48 J/m 2 (friction).