Preparation of proton-beta/beta"-aluminas is realised by an ion-exchange process under hydrothermal conditions using dilute acetic acid or sulphuric acid as the medium, and the corresponding alkali beta/beta"-aluminas as the starting materials. The optimum conditions involved in this process is around 425-450 K under the autogenerated pressure below 50 MPa, and a duration of 2-10 h. At higher temperatures and pressures or extended duration, proton-a-alumina converts to boehmite, gamma-AlOOH, in the same medium. This method yields products with a variable range of composition with [H2O]:[Al2O3] = 1:5 to 1:17. The resulting products convert to a-alumina when heated to temperatures above 1273 K under ambient pressures. High resolution transmission electron microscopy (HRTEM) indicates the variable contents of microsyntactic defects and stacking faults with the changing composition. This study also shows the retention of the p-alumina structure during the displacement of alkali by hydroxonium ions under optimum hydrothermal conditions.