Aqueous dispersion containing transition aluminium oxide as only solid and a dispersant, in which--the transition aluminium oxide is present in the form of aggregates of primary particles, the content of transition aluminium oxide in the dispersion is from 40 to 65% by weight, the aggregates in the dispersion have a mean aggregate diameter of less than 100 nm,--the dispersant contains polyaluminium hydroxychloride, polyaluminium hydroxynitrate and/or polyaluminium hydroxysulphate, the dispersion has a pH of from 3 to 5.