The surface of precipitated calcite particles was modified by reaction with phosphonic acids (phenylphosphonic acid and dodecylphosphonic acid) in organic solvents, tetrahydrofuran (THF) and ethanol. The reaction kinetics and adsorption isotherms were determined. Under appropriate conditions, dense phosphonate monolayers were deposited at the surface of the CaCO3 particles. The nature of the grafted species was investigated using 31P and 1H Magic Angle Spinning (MAS) solid state NMR spectroscopy, FTIR spectroscopy and N2 physisorption. These monolayers were found to increase the hydrophobic character of CaCO3 surfaces, especially in the case of dodecylphosphonic acid. The unwanted formation of a bulk crystalline calcium phosphonate phase by dissolution–precipitation was observed only in the reaction of phenylphosphonic acid with CaCO3 in THF, and it could be largely avoided by working at low concentration of the phosphonic acid and/or minimizing reaction times to less than 3 days.