An important factor in producing titanium dioxide pigments by the sulphate process is the reactivity of the Ti raw material with sulphuric acid. This study shows that Australian ilmenite concentrate contains poorly soluble titanium dioxide-bearing phases with an unfavourable textural distribution. The concentrate is composed of ilmenite and its alteration products, and a mixture of several alteration phases is present in most single grains. Alteration of the ilmenite grain advances to the formation of poorly soluble phases along grain boundaries and fractures that prevent the acid from reaching the soluble ilmenite. The textural relationships of the different phases prevent the liberation of highly altered phases from unaltered or slightly altered phases. The trace element (Cr, V, Si, Al, etc.) concentration increases along with alteration. Most of the deleterious materials are concentrated in the fine fraction of the concentrate.