A micromechanical method to measure and analyze interfacial debonding of single fibers in transverse tension was developed. The damage process was observed in-situ by optical microscopy. Two glass fiber/epoxy (GF/EP) systems were studied with different fiber surface treatment. Results from calculations of the interfacial fracture toughness for Mode I and Mode II failure are presented. The theoretical analysis is based on a modification of Toya's stress analysis of a debonded circular inclusion, calculation of the work required to close the crack and a mixed-mode failure criterion