The invention relates to an aqueous composition comprising (a) polymers having primary amino groups and/or amidine groups to a combined content for these groups of ≧1.5 meq/g of polymer, and (b) 0.01 to 50 mol % of 1,4-cyclohexanedione (b) based on the combined amount of primary amino groups and amidine groups of the polymers, wherein the pH of the aqueous composition is ≦6, and further to its use as strength enhancer and to a method of producing paper and board, employment of the aqueous composition and also the paper and board thus obtained.