The acid–base and complexing properties of diastereoisomers of phosphonodipeptides containing glycyl, L-alanyl, L-leucyl and L-phenylalanyl and terminal 1-aminoethylphosphonic acid residues were studied pH-metrically at 25 °C and at an ionic strength of 0.1 mol dm^–3(KNO₃). The protonation and complex-formation stability constants with Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ indicate the presence of the same types of complexes and the same tendencies as in the aminomethylphosphonic acid series. In contrast to the common dipeptides, the differences found for diastereoisomers are much higher, probably due to stronger interaction between the hydrophobic and hydrophilic parts of the molecule. Crystal structure determinations confirmed the absolute configurations of S,S-PhCH₂CH(NH₂)-CONHCH(Me)PO₃H₂ and S,R-PrⁱCH₂CH(NH₂)CONHCH(Me)PO₃H₂. The distances and bond angles found show that the peptide bond is not influenced by the phosphonic group.