The conditions under which vanadium is rendered insoluble by silica in the alkali-roast process have been investigated starting with both slag and reagent-grade materials. The inert phase is a glass formed slowlyfrom soluble NaV03 produced at lower temperatures. It contains approximately 30 per cent V in a partly reduced state; iron is not a principal component. Some of the vanadium in the inert phase is slowly returned to a soluble form by heating in an oxidizing atmosphere below the temperature at which the glass is formed (750C). As the reaction leading to the required, soluble form of vanadium is also slow between 600 and 750 optimization and control of the temperature profile and atmosphere in the furnace, and the residence time of the solids, are necessary to ensure high vanadium extraction during subsequent leaching.