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In this study, the partitioning of fluoxetine, an antidepressant of selective serotonin reuptake inhibitor class into a mixture containing anionic and zwitterionic lipid vesicles was evaluated using second derivative spectrophotometry. The partition coefficients (Kp) of fluoxetine into the large unilamellar vesicles (LUVs) composed of zwitterionic 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) containing 0 mol%, 10 mol%, 20 mol%, and 30 mol% of anionic 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) were measured in HEPES buffer at pH 7.4. The result revealed that when more negatively charged lipids incorporated into the LUVs, the condensing effect on the binary phospholipid membrane impeded the partitioning of positively charged fluoxetine, resulting in the decrease in the Kp values. This study adds a deeper understanding of how antidepressant fluoxetine exerts its effect on anionic-containing biological membranes, shedding light onto drug delivery systems in the pharmaceutical field.