Coated tablets are prepared by applying to a core of active material, at least one layer of a coating composition made up of a film forming aqueous synthetic resin dispersion and from 2-50% by weight of a water or alkaline soluble material and thereafter permitting the coating composition layer to dry. The resulting coated tablet has a core surrounded by a continuous porous matrix of synthetic resin formed from the aqueous dispersion which is insoluble in water and insoluble in the gastrointestinal tract. The pores of the continuous resin matrix are filled with a discontinuous particulate material which is water or alkaline soluble. The coated tablet initially is air and moisture tight.