The strength transverse to the fibre (or the transverse fracture strain) of CFRP is directly related to the fibre/matrix bond strength. This enables the determination of a fibre/matrix bond strength based on transverse fracture data. In crossply laminates, the transverse ply usually fails many times, so that principally these experiments deliver a treasure of data. A method, developed to make use of these data, produces a Weibull distribution for transverse fracture based on the data of, basically, a single specimen. A model developed to separate the influences of defects, matrix ductility and constraint finally leads to a procedure to determine the strain at interphase (or interface) failure. The usefulness of the model and the determined strain at interface failure is demonstrated with several examples, such as the influence of constraint, of test temperature and of the fibre surface treatment on transverse cracking and the fibre/matrix bond strength.