AbstractProperties of interface in unidirectional composite materials can be studied “in situ” by dynamic mechanical measurements: analysis of relaxations and appearance of damage in shearing. This methodology is applied on composite materials based on an epoxy matrix and carbon fibers with different surface treatments (untreated, oxidized and sized). Mechanical spectroscopy displays modifications of epoxy network with the fiber treatment and evidence for an interphase in the case of sized fibers. Dynamic shearing test clearly shows large differences of behavior with the surface treatme