Geoid height anomalies derived from SEASAT altimeter data have been analyzed across fracture zones and over ocean ridges in two limited regions of the South Pacific (Eltanin fracture zone system; East Pacific Rise) and southeast Indian Ocean. Observed geoid height-age and geoid height derivative (with respect to age)-age relationships have been established. Comparison with theoretical relationships computed for the plate model of lithospheric cooling permits an estimation of the thermal parameters entering into the model. Two quantities can be derived; the product α&kgr;Tm (α, thermal diffusivity; &kgr;, volume coefficient of thermal expansion; Tm , bottom boundary temperature) and the thickness H of the plate. The best fitting values for the South Pacific and the southeast Indian Ocean are α&kgr;Tm =0.52¿10-3 cm2 s-1 and H in the range 50-70 km for ages less than 30 m.y., whereas for larger ages (≥30 m.y.), geoid observations are better explained by a larger H value, in the range 70-90 km. The depth-age relationship observed in the South Pacific region is consistent with these parameters. On the other hand, data for the northeast Pacific argue rather for a lithospheric thickness larger than 100 km. This suggests that local and regional variations exist in the thermal properties of the oceanic lithosphere.