A model, based on fluorescence data, is developed for the poly(acrylic acid)-assisted transport of doxorubicin, a cationic antitumor drug, through a bilayer membrane. Accordingly, the doxorubicin binds to the poly(acrylic acid) via electrostatic polymer-drug interactions plus drug-drug stacking within the complex. This complex associates with neutral egg lecithin vesicles by means of hydrophobic attraction between the doxorubicin and the vesicle bilayers. In the process, the doxorubicin "destacks", providing a fluorescence change that can be monitored. Finally, the doxorubicin enters the vesicle interior which has been imparted with an acidic pH to protonate the doxorubicin and thus, in a second stage, yield an additional fluorescence change that can also be detected. A portion of the poly(acrylic acid), now devoid of doxorubicin, then leaves the outer vesicle surface and enters to external solution.