We present a new way to estimate the thermal diffusivity and the thickness of the oceanic lithosphere based on admittances computed from geoid and bathymetry profiles which roughly parallel the ridge. This method appears to operate in the medium wave band (500--1500 km), and may be of help in remote oceanic areas where only poor bathymetry is available. We apply the method to 32 profiles on each flank of the South Atlantic spreading ridge between the equator and 35¿S, and obtain the solution domain for both the plate thickness and the thermal diffusivity. Anomalously high admittance values obtained for the west flank (at wavelengths around 700 km) may be due to deeper asthenospheric density heterogeneities. ¿ American Geophysical Union 1990