In this study, the surface of a composite prepreg was treated using an atmospheric pressure plasma in an attempt to improve the fracture toughness of a co-cured joint system. Three gas mixtures were investigated; helium, helium/nitrogen, and helium/oxygen. The processing parameters of the system were varied to obtain the maximum increase in surface energy of the prepreg. A He/O-2 plasma was found to be the most efficient treatment, giving the largest increase in surface energy in the shortest time. Co-cured joints were then fabricated using prepreg that had been treated with various plasmas. A modest 15-18% increase in the Mode I fracture toughness was achieved. However, the locus of failure remained interfacial. It was also observed that a He/O-2 plasma treatment could be detrimental to joint toughness for long treatment times.