Australia has had a long and complex history of intense surficial weathering and this has led to the extensive development of laterites, bauxites and deeply weathered profiles of a variety of ages across the continent 1,2. A detailed knowledge of the age(s) of such weathering phenomena is of great importance for the reconstruction of Australia's post-Palaeozoic geomorphic and palaeoclimatic history. However, in many instances, absolute geo-chronological information is not obtainable using conventional techniques. Here we describe and evaluate a new method that can provide an estimate of the age of intensely weathered profiles based on the oxygen isotope composition of minerals developed by deep-weathering/lateritization processes. This method allows the definition of four ages of weathering in eastern Australia from the late Palaeozoic to the present and suggests that some profiles previously thought to be late Cretaceous or Tertiary in age may be of pre-late Mesozoic age. The oxygen isotope data indicate that the minerals used in this study have retained their original isotopic composition for periods of up to 250 million years, despite the continued presence of meteoric waters of progressively higher 18O content during this interval. �� 1988 Nature Publishing Group.