In the paper, the influence of the addition of different ion-pair reagents — sodium-1-heptane sulfonate (HS), tetrabutylammonium chloride (TBA), and bis(2-ethylhexyl)hydrogen phosphate (HDEHP) — to the mobile phase on the velocity of the electroosmotic flow (EOF) of the mobile phase in pressurized planar electrochromatography (PPEC) was examined. During the experiments, glass-based high-performance thin-layer chromatography (HPTLC) RP-18W chromatographic plates were used as the stationary phase. The mobile phase was composed of acetonitrile and water in the ratio 25:75 (v/v) with acetic buffer (4 mM) and with or without ion-pair reagent. Our research shows that ion-pair reagent in the chromatographic system significantly affects the value of zeta potential and the value of the velocity of the electroosmotic flow of the mobile phase. The results of our study also demonstrate that, in PPEC, as in other chromatographic techniques, a small addition of ion-pair reagents affects the selectivity of the separation.