Voltage gradient partial-filling affinity capillary electrophoresis (VGPFACE) is used to determine binding constants between carbonic anhydrase B (CAB, E.C.184.108.40.206) and arylsulfonamides, and vancomycin (Van) from Streptomyces orientalis and teicoplanin (Teic) from Actinoplanes teicomyceticus and D-Ala-D-Ala terminus peptides. Two variations of VGPFACE are described herein. In the first technique, the capillary is partially filled with ligand at increasing concentrations followed by a sample containing receptor and two noninteracting standards and electrophoresed in buffer using a voltage gradient that increases from 0 to 25 kV over the duration of the experiment. Upon continued electrophoresis, zones of solution overlap, and equilibrium is established between the ligand and receptor, causing a shift in the migration time of the receptor with respect to the noninteracting standards. This change in migration time is utilized for estimating a binding constant (K(b)). In the second technique, voltage gradient partial-filling multiple-injection ACE (VGPFMIACE), a multiple-injection sequence is used whereby the capillary is partially filled with ligand at increasing concentrations, a noninteracting standard, three or four separate plugs of receptor each separated by small plugs of buffer, and a plug containing a second noninteracting standard; this is then electrophoresed in buffer with a similar voltage gradient. Upon continued electrophoresis, a similar equilibrium is established and a value for K(b) is obtained for the interaction. The VGPFACE technique expands the functionality and potential of ACE as an analytical tool to examine various receptor-ligand interactions.