>A technique for obtaining age estimates for regolith profiles in Australia, based on the oxygen���isotope composition of the clay mineral assemblage in a profile, is applied to a variety of regolith profiles and kaolinitic sediments from across Australia. Excluding monsoonal regions in the north of the continent, it is possible to distinguish profiles formed in the Late Mesozoic���Early Tertiary (��18O values between +15 and +17.5%����) from profiles formed in post���mid���Tertiary times (>+17.5%��). In addition it is concluded that there remain widespread remnants of a deep���weathered regolith which developed in pre���Late Mesozoic (Early Cretaceous or Jurassic?) times when Australia was at high latitude. The low ��18O values associated with clays formed in pre���Late Mesozoic times (+10 to +15%o) suggest that deep weathering took place in a cool to cold and presumably humid climate, contrary to the traditional belief that deep weathering requires tropical to subtropical temperatures. The formation of deep���weathered profiles at high latitude in a comparatively cold climate may be linked in part to higher past atmospheric CO2 levels. �� 1993 Taylor & Francis Group, LLC.