In standard models for computing oceanic lithosphere thickness, the lithosphere's thermal diffusivity is one of the basic parameters required and present estimates have a substantial uncertainity. Extending the work of Crough [1979>, Lister [1982> and Gibert and Courtillot [1990>, we propose a new method for the determination of thermal diffustivity of oceanic lithosphere in the near-ridge crest domain. The method requires geoid height and depth data, both of which reflect wavelengths associated with the lithospheric cooling process and expressed as a function of seafloor age. Over the Southwest Indian Ridge, the thermal diffusivity so found [(0.55¿0.17)¿10-2cm2s-1> is consistent with data from the South Atlantic. Differences in diffusivity calculated on either side of the ridge and caused by asymmetrical subsidence are possibly linked to a differential thermal regime beneath the ridge and/or to the general motion of the ridge. Our findings are interpreted in terms of a lithospheric boundary layer less than 14age1/2 kmMyr-1/2 thick and initial temperature between 1050 and 1390 ¿C. ¿ American Geophysical Union 1992