On-line affinity capillary electrophoresis-electrospray ionization-mass spectrometry (ACE-MS) was used for the simultaneous measurement of multiple binding constants of an all-D-tetrapeptide library to the model receptor, vancomycin. Determination of Kd values for the 19 peptides of the form Fmoc-DXYA is demonstrated. The data are compared with the results obtained for individual compounds using ACE-UV, and good correlation between the two detection methods is shown. Simultaneous determination of multiple Kd values by ACE-MS is achieved in one set of experiments, whereas only one Kd value can be obtained by ACE-UV during the same time. ACE-MS measures multiple binding constants in solution in a fast and reliable manner using femtomole amounts of samples.