A mechanism based on the adsorption of the phosphoric acid on the external surface of the CaCO3 solid phase followed by reaction of surface species with phosphoric acid and further transfer of the formed Ca(H2PO4)(2) in the solid-liquid interface for the dissolution of CaCO3 particles in a phosphoric acid solution is assumed. The reaction of surface species with phosphoric acid was found to be the rate-determining step. The dissolution rate of CaCO3 particles is decreased with. the increase of the particle size and is affected dramatically by treatment. The dissolution of untreated particles is accompanied by intense breakdown of the particles, and the rate and extent of;breakdown depends on the particle size. Adequate pretreatment with steam makes the CaCO3 particle resistant to breakdown, and the dissolution rate becomes very slow.