ScSp arrivals, resulting from ScS-to-P conversions at upper boundaries of downgoing plates in subduction regions, have been identified on Japanese and South American seismograms. Analysis of the ScSp arrivals provides information about the sharpness and the location of these boundaries. It is inferred from ScSp arrivals that the boundaries are characterized by velocity contrasts of at least 5 per cent over transition zones of at most a few kilometers. In general, locations of the boundaries inferred from ScSp observations agree with locations inferred from seismicity except beneath central Peru. In central Peru the seismicity at depth is diffuse. It has been proposed on the basis of that seismicity that the downgoing plate dips at a gentle 10°; the ScSp observations, however, suggest that the Nazca plate has a dip of ∼30°.