Free solution capillary electrophoresis (CE) is a powerful separation technique for the characterization of diblock copolymers. In this work, four series of double-hydrophilic anionic and cationic block copolymers, namely, poly(acrylamide)block-poly(acrylic acid), poly(acrylamide)-block-poly((3-acrylamidopropyl)trimethylammonium chloride), poly(ethylene oxide)-block-poly(acrylic acid), and poly(poly(ethylene glycol)methyl ether acrylate)-block-poly(acrylic acid), were synthesized by reversible addition-fragmentation chain transfer polymerization and characterized by CE. The electrophoretic mobility distributions of the copolymers were transformed into distributions of composition ratio by introducing a retardation parameter, X-exp, that represents the hydrodynamic drag retardation due to the neutral block of the copolymer. A linear correlation between X-exp and the ratio of the degrees of polymerization of each block was experimentally established and was consistent with the model of electrophoretic mobility of composite macromolecules with hydrodynamic coupling. Finally, the comparison of the distributions between the different copolymer families was significantly improved by considering the distributions in composition ratio rather than the electrophoretic mobility distributions because it takes into account the differences in solvation, expansion, and drag force according to the chemical nature of the blocks.