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As two emerging pollutants of great concern, microplastics (MPs) and antibiotics inevitably cooccur in various aquatic environments and interact with each other, impacting the fate and ecological risks. Aging obviously complicates their interaction and deserves further study. Therefore, the adsorption-desorption behaviors of ciprofloxacin (CIP) onto polystyrene (PS) fragments with various aging extent were investigated, and the key physiochemical properties influencing the interaction and the interaction mechanisms were clarified by redundancy analysis, FTIR and XPS spectra. The physicochemical properties of PS MPs were significantly changed with aging time, and the morphological and chemical changes seemed to occur asynchronously. The adsorption of CIP onto the pristine PS MPs relied on physisorption, especially the ion-involving electrostatic and cation-π interaction. Due to the hydrogen bonding formed by the C–OH, C O, and O–C O groups of PS and CIP, the adsorption capacities of the aged PS MPs were greatly increased. The desorption efficiency of CIP from MPs in the gastric fluid was closely related to the solution ionic strengths, C–OH and C O groups of MPs, while that in the intestinal fluid was associated with O–C O groups of MPs. The different impact factors could be well described by the differences in the chemical components and pHs of the simulated gastric and intestinal fluids. This study gives a comprehensive understanding of the adsorption-desorption behaviors of antibiotics onto MPs at a molecular level and indicates that MPs could act as Trojan horses to transport antibiotics into aquatic organisms. [Display omitted] • Redundancy analysis identified MPs' key properties affecting adsorption/desorption. • Hydrogen bonding, electrostatic and cation-π interaction were crucial for CIP adsorption. • The C–OH, C O, and O–C O groups of aged MPs were involved in the hydrogen bonding with CIP. • Desorption of CIP in the gastric fluid relied on the C–OH and C O groups of MPs. • Desorption of CIP in the intestinal fluid was related to the O–C O groups of MPs. [ABSTRACT FROM AUTHOR]