Diatomite filter-aids coated with polyeletrolytes such as C.M.C, gelatine and polyacrylamide is adsorb-ed, were used in the vacuum precoat filtration of dilute ink and coffee slurries. The amount of adsorbed polyeletrolytes increased proportionally with the ratio of polyelectrolyte/deatomite, and the relation of adsorption equilibrium concentration and adsorption amount is well fitted with Freundlich equation. The polyelectrolyte adsorption on diatomite enabled to filler out ultrafine particulate matter which would otherwise call for much finer grdes of diatomite. The turbidity of filtrate was improved with increasing amount of polyelectrolyte adsorbed. The polyelectrolyte adsorption on diatomite decreased the filtration rate.