The effect of propylene glycol (PG) on transdermal flux under current was investigated using conventional in vitro iontophoresis methodology. The results were evaluated to explain how PG affects the electroosmotic volume flow (EVF) and electromigrational flux through skin. As a marker molecule for the direction and magnitude of EVF, a non-charged neutral molecule, acetaminophen (AAP), was used. At pH 7.4, the direction of EVF was from anode to cathode. During anodal and cathodal current application, PG decreased AAP flux and this decrease was proportional to the concentration of PG, indicating that the presence of PG in the medium decreased the EVF. This decrease is likely due to the decrease in dielectric constant of the medium and the increases in medium viscosity by the addition of PG. The increase in AAP solubility and the viscosity of the medium by PG may also contribute to the decrease in diffusional flux. The magnitude of EVF was estimated to be about $4.2{mu}l/cm^2h$. The effect of PG on the flux of a positively charged drug, donepezil hydrochloride (DH), was further investigated using pH 4.6 phosphate buffer solution. The permselectivity of skin in this solution was also investigated and revealed that the isoelectric point of hairless mouse skin is higher than pH 4.6. Anodal delivery showed much higher flux than cathodal and passive flux, indicating that electromigration is playing the major role for DH flux. As the concentration of PG increased, anodal flux of DH decreased. The main reason for this decrease in electromigration is likely due to the increase in medium viscosity. These results and discussions clearly suggest that the incorporation of frequently used organic cosolvents and penetration enhancers into the iontophoretic formulation should be carefully chosen with a thorough investigation for their effect on flux. Overall, these results provided further mechanistic insights into the role of electroosmosis and electromigration in flux across skin, and how they can be modulated by organic cosolvent, PG.