A functionalised solid product containing sulphonate or sulphonic acid groups is prepared from an inorganic oxide containing surface hydroxyl groups, e.g. silica by reacting the latter in a first stage with a haloalkyl alkoxy- or arylox- ysilane. The first stage product is then reacted with an aqueous solution of an inorganic sulphite to give a product containing sulphonate groups. These can be converted to sulphonic acid groups by an optional third stage treatment with dilute mineral acid. The final products are suitable for use as adsorbent materials or catalyst supports.