Abstract : Adsorption of antibiotics at mineral surfaces has been extensively studied over the past 20 years, yet much remains to be learned on their interfacial properties and transformation mechanisms. In this study, interactions of Ciprofloxacin (CIP), a fluoroquinolone antibiotic with two sets of synthetic nanosized hematite particles, with relatively smooth (H10, 10-20 nm in diam.) and roughened (H80, 80-90 nm in diam.) surfaces, were studied by means of liq. chromatog. (LC), mass spectrometry (MS), and spectroscopy (vibration and X-ray photoelectron). Attenuated Total Reflectance Fourier Transform IR (ATR-FTIR) spectroscopy provides evidence for inner-sphere bidentate complex formation of CIP at hematite surfaces in 0.01 M NaCl, irresp. of pH and particle size. ATR-FTIR spectroscopy also revealed that the sorbed mother CIP mol. decayed to other surface species over a period of at least 65 h. This was supported by the detection of three daughter products in the aq. phase by LC/MS. The appearance of NH3+ groups during the course of these expts., revealed by cryogenic XPS, provides further evidence that CIP oxidn. proceeds through an opening of piperazine ring via N-dealkylation. Addnl. in vacuo FTIR expts. under temp.-programmed desorption also showed that oxidn. of sorbed byproducts were effectively degraded beyond 450 °C, a result denoting considerably strong (inter)mol. bonds of both mother and daughter products. This work also showed that rougher, possibly multidomainic particles (H80) generated slower rates of CIP decompn. but occurring through more complex schemes than at smoother particle surfaces (H10). This work thus uncovered key aspects of the binding of an important antibiotic at iron oxide surfaces, and therefore provided addnl. constraints to our growing understanding of the fate of emerging contaminants in the environment.