Self-dimerizations of twenty three aromatic carboxylate and sulfonate ions from their electrophoretic mobilities in aqueous solution were estimated by capillary zone electrophoresis (CZE). The magnitudes of the self-dimerizations ascribed to pi-pi interactions of these aromatic anions were determined by CZE as dimerization constants (KD). Although the largest KD value of 1.2 dm3 mol(-1) for 9-anthracenecarboxylate ion (9-AC) in these aromatic anions was found, almost all of the KD values were zero, or near to zero. It was found that the pi-pi interactions of the aromatic anions were relatively small at zero ionic strength, in which the contribution of an ionic association between the cation and aromatic anions could be excluded from the KD values, since the contribution of the electric repulsion between the aromatic anions on the KD values was large. The relatively large KD value of 9-AC caused that it electro-migrates as its planar shape, and has an anthracene ring of a largely hydrophobic aromatic ring.