The kinetic method has been used for enantiodiscriminating important chiral residues from post-translationally modified proteins, i.e., the O-phospho .alpha.-amino acids, and some chiral .alpha.-amino acid mimics, i.e., the .alpha.-aminophosphonic acids. Their proton affinities (PA) have been evaluated using the same method in its extended form which allows the est. of the entropy contributions on the kinetic measurements. The kinetic approach is based on the competing collision-induced decompn. (CID) of suitable proton-bound complexes between the P-contg. mols. and suitable ref. compds., e.g., .alpha.-amino acids. The investigation was carried out in a QqQ instrument as a function of the collision energy (4-14 eV). The proton-bound complexes were generated in an electrospray ionization (ESI) source from calibrated water/methanol solns. of the two components. The exptl. results have been discussed and compared with theor. calcns.