A continuous process for the hydrothermal conversion of a solid starting iron compound selected from the group of iron oxides, iron hydroxides, iron oxyhydroxides, and mixtures thereof into a solid product iron compound with different physical, chemical, and/or structural properties. The process comprises the steps of a) dispersing the solid starting iron compound in a liquid thus forming a suspension, and b) feeding the suspension continuously though one or more agitated conversion vessel(s), in which vessel(s) the solid starting iron compound is converted into the solid product iron compound under hydrothermal conditions.