Seismic refraction surveys have been conducted over the convergent plate boundaries of the New Hebrides and Tonga, in a joint programme by the Office de la Recherche Scientifique et Technique Outre-Mer (Centre de Noum&) and the University of Texas (Institute of Marine Science). From 24 profiles taken during the EVA II, EVA IV and EVA VI1 cruises, it appears that 1. In spite of overall similarities, differences in the shallow structure of the two arcs can be seen. In the New Hebrides the low-velocity layers are much thicker than in Tonga, especially in the lower part of the inner slope of the trench. This difference in thickness can be correlated with the differences in thickness of the transition and oceanic layers of the crust of the dipping plate. 2. Refraction leaves uncertainty as to the structure at depth, particularly as to the joining of the arc and the back-arc basins. The existence of 7.6-7.7 km/s velocity layers complicates the interpretation in classical terms of crust and mantle. One possible interpretation of the evolution of the crust under the island arcs could be a thinning-down in time.