A contribution to the description of electrokinetic effects on the pH boundary formed by sodium borate pH 9.5 and sodium phosphate pH 2.5 electrolytes for on-line preconcentration of weak acids is presented in this article. Simulations of electrokinetic injections together with experimental studies using contactless conductivity detection verified that the preconcentration is induced mainly by dissociation changes of analytes on the pH boundary and transient ITP state. Moreover, a study of the addition of organic solvent to the injection electrolyte was performed with impressive results. Subnanomolar LODs of hydroxybenzoic acids were achieved with 80% of methanol in the injection electrolyte which represents more than 70 000-fold preconcentration in comparison with classical CZE method.