To determine the mechanism of acid-leaching of ilmenite to ultimately forming rutile, we have carried out an experimental study of ilmenite alteration in autoclaves at 150°C in HCl solutions. The resulting products were studied by X-ray diffraction, scanning electron microscopy, electron microprobe and Raman spectroscopy. In some experiments the solution was initially enriched in 18O and the distribution of the isotope in the alteration products mapped from the frequency shift of cation—oxygen stretching bands in the Raman spectra. The alteration begins at the original ilmenite crystal surface and has also taken place along an intricate branching network of fractures in the ilmenite, generated by the reaction. Element-distribution maps and chemical analyses of the reaction product within the fractures show marked depletion in Fe and Mn and a relative enrichment of Ti. Chemical analyses however, do not correspond to any stoichiometric composition, and may represent mixtures of TiO2 and Fe2O3. The fracturing is possibly driven by volume changes associated with dissolution of ilmenite and simultaneous reprecipitation of the product phases (including rutile) from an interfacial solution along an inward moving dissolution-reprecipitation front. Raman spectroscopy shows that the 18O is incorporated in the rutile structure during the recrystallization. Throughout the alteration process, the original morphology of the ilmenite is preserved although the product is highly porous. The rutile inherits crystallographic information from the parent ilmenite, resulting in a triply-twinned rutile microstructure. The results indicate that the ilmenite is replaced directly by rutile without the formation of any intermediate reaction products. The reaction is described in terms of an interface-coupled dissolution-precipitation mechanism.