Low-energy collisionally activated decomposition (CAD) and unimolecular decomposition of the [M+H]+ ions for X-L-Pro-L-Phe, where X is L-Ala, D-Ala, L-Asp, or D-Asp, allow easy differentiation between the LLL and DLL diastereomers. Tandem mass spectrometric (MS/MS) studies of the [M+H]+ ions formed by fast-atom bombardment (FAB) at various ion kinetic energies (Elab values) on a hybrid tandem instrument produced ions of different intensities for the diastereomers. The ratio of NH3 to H2O loss is 0.3 for the L-Ala peptide but 1.7 for the D-Ala isomer at 5 eV. In some L-Ala spectra, the [M+H-NH3]+ ion does not appear at all. The y2 ion is up to twice as abundant in the L-Ala spectra as in the D-Ala, while the b2 ion is somewhat more abundant for CAD of the D-Ala peptide for most collision energies investigated. The D-Asp peptide produces a b2 ion that is more than half-again as abundant as in the case of the L-Asp isomer, and an [M+H-H2O]+ ion that is up to twice as abundant in the D-Asp CAD spectra as in those of the L-Asp. The y1, a2, and phenylalanine immonium ions are each up to twice as abundant in the L-Asp spectra as in those of the D-Asp isomer. The major differences are correlated with force-field calculations on hydrogen-bonded tautomers.