The results of a seismic refraction profile across the Southern Andes at 39°S from the Chilean Pacific coast to the Argentinean Neuquén Basin are presented here. A 2-D crustal velocity model was derived from traveltime forward modeling of the correlated phases on shot-sections from shot-points in the Pacific Ocean and in the Chilean Main Cordillera. The crustal velocity model is characterized by lower average velocities in the fore-arc than in the arc. A typical continental Moho was not observed under the fore-arc, under the arc the Moho could only be inferred from teleseismic receiver function studies. The oceanic Moho was observed down to ca. 45 km depth under the continental fore-arc. Using additional structural and seismic data, the velocity model was extrapolated to the west and to the east, and this model was converted into a rough density model. The gravity, computed from this density model, is consistent with the free-air and Bouguer gravity. A crustal thickness of ca. 40 km is inferred for the magmatic arc. Gradually decreasing average velocities of the continental crust from east to west are proposed to be due to the influence of the intrusion of the North Patagonian batholith in the east and of decreasing age of the accretionary wedge together with underplating of subducted sediments at the base of the continental crust in the west.