Mixtures of high molecular weight polyacrylamides in the form of a water-in-oil polymer dispersion and containing in the inner phase from 99.99 to 95% w/w of a cationic polyacrylamide and from 0.01 to 5% w/w of an anionic polyacrylamide or from 99.99 to 95% w/w of an anionic polyacrylamide and from 0.01 to 5% w/w of a cationic polyacrylamide, all percentages being based on the total amount of polymer. The said polymer mixtures are particularly effective retention aids and flocculants and are preferably used in the manufacture of paper as pulp additives.