The conditional binding constants for a bis-guanidinium-like receptor and a series of dicarboxylate ligands have been determined in two buffer/solvent systems, namely 25 mm ammonium acetate/1% acetic acid in acetonitrile/methanol (7:3 v:v) and 30 mm N-methyl morpholine/15 mm methanesulfonic acid in acetonitrile/methanol (9:1 v:v). The latter buffer has not been applied before in capillary electrophoresis. The binding constants in both solvent systems decrease as the dicarboxylate length increases. The binding constants are larger in the less competitive N-methyl morpholine buffer. The dicarboxylates associate only weakly with a dicationic analog of the receptor, p-xylyl trimethylammonium, which is not a hydrogen bond donor.