Cationic polyvinyl acetals are prepared by copolymerizing one or more cationic N-alkyldiallylammonium salt monomers with one or more vinyl esters of branched or unbranched carboxylic acids having 1 to 15 carbon atoms, hydrolyzing the resulting copolymers to copolymers containing .gtoreq.50 mol % vinyl alcohol units, and acetalizating the vinyl alcohol units with one or more aliphatic aldehydes having 1 to 15 carbon atoms, their acetals, and/or hemiacetals, wherein the copolymerization takes place in a mixture of water and monovalent aliphatic alcohol having a water content of 2% to 35% by weight.