The enthalpies of solution of aminomethylphosphonic acid (Gly(P)), 1-aminoethylphosphonic acid (Ala(P)), and 1-aminopropylphosphonic acid (Hala(P)) have been measured in water and aqueous urea solutions at 298.15 K. From the obtained experimental results, the standard enthalpies of solution of aminophosphonic acids (A(P)) in water-urea solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan-Mayer's formalism. These values were interpreted in the terms of the hydrophobic effect of the side chains of aminophosphonic acids on their interactions with a molecule of urea in water.