The present study shows the relation between the surface properties of composite materials, treated with common surface preparation methods, and the mechanically measured bond strengths as quoted from lap-shear tests. The surface properties are studied by roughness measurements, surface free energy assessment, X-ray photoelectron spectroscopy and scanning electron microscopy. The procedures followed, reveal the measure of significance of the mechanical interlocking, kinetics of wetting, chemical reactivity and intermolecular adhesion of the interfaces. It is shown that the governing adhesion qualities determine significantly the fragmentation process and the strength of the joints alongside the load transfer mechanism that is analysed by a simple finite element model. Based on the results, an emphasis is given on elucidating the difference between the intrinsic interfacial adhesion strength and the measured bond strength.