SONNE cruise SO-85 was designed to study the variability of the oceanic crustal structure along flow-line profiles over Early Tertiary through Early Cretaceous old crust in the Cape and Argentine basins of the South Atlantic. Between 21th February and 20th April 1993 deep imaging seismic reflection data in parallel with magnetic, gravimetric and hydroacoustic measurements have been collected along a 1,405 km long flow-line traverse across the Cape basin, and along three flow-line traverses across the Argentine basin with a total length of 4,255 km. A special geophysical survey was carried out on the Argentine continental margin. Although bad weather hampered the geophysical measurements in the Argentine basin, some new and intriguing observations were made: (1) the presence of an elongated wedge of seaward dipping reflectors, often associated with the distinct magnetic anomaly 'G', was confirmed on both, the South African and Argentine continental margins; (2) the sequence of seaward-dipping reflectors consists of two wedges. The younger wedge rests partly on the older one; (3) the surface of the oceanic crust lies more deep in the Argentine basin than in the Cape basin; (4) the structure of the oceanic crust is more variable in time and space within the Argentine and Cape basins than within the Angola and Brazil basins; (5) despite this, it appears that the oceanic crust generated during the younger half of the Cretaceous magnetic quiet period up to C 33r is characterized by relative high reflectivity in lower crustal levels; (6) high-amplitude magnetic anomalies C 33 through C 34 are apparently correlatable with oceanic crustal segments characterized by common seismic features, e.g. a smooth and scarped surface of the oceanic crust; (7) an elongated, positive gravity anomaly exists at about 58°E between 43.5°E and 47°E.