Abstract: Acid reactive mineral particles are treated by dissolving certain hydrophilic organic polymeric compounds in the interparticulate water of an aqueous dispersion of the mineral particles and adding an interacting chemical to insolubilize the polymeric material and thereby form stable particle plus polymer agglomerates having a decreased rate of attack by acid. The acid protected particle plus polymer agglomerates are formed with a paper furnish into a paper sheet. A particularly preferred mineral is calcium carbonate. Suitable polymeric compounds are starch, sodium alginate, casein and alkali- solubilized latices. Sometimes it may be advantageous to use organic polymeric compounds in combination with one another.