The enthalpies of solution of 1-aminobutylphosphonic acid (Nval(P)), 1-amino-2-methylpropyl-phosphonic acid (Val(P)), 1-amino-3-(methylthio)propylphosphonic acid (Met(P)), N-methyl-1-aminopropyl-phosphonic acid (Me-Hal(P)), and N,N-dimethyl-1-aminopropylphosphonic acid (Me-2-Hal(P)) in water and aqueous urea solutions were measured within the molality range from (0.5 to 4) mol (U)kg(-1) (H2O) at a temperature of 298.15 K. The results obtained were used to determine the standard enthalpies of solution, whose values were then used to calculate the enthalpic heterogeneous pair (phosphonic amino acid urea) interaction coefficients.