Purpose <p>The presence of dissolved organic matter (i.e., fulvic acid (FA)) can affect the sorption of antibiotics onto nanoparticles, thereby influencing their co-migration with nanoparticles in the environment.</p> Materials and methods <p>Through the sorption batch experiments and dialysis batch experiments, combined with Fourier transform infrared spectroscopy, X-rays diffraction and other characterization methods, the influencing mechanisms of FA binding on sorption of ofloxacin (OFL) onto nano-montmorillonite (nano-MONT) and carbon nanotubes (CNT) were investigated.</p> Results <p>FA and OFL had strong reactivity in aqueous solutions, meanwhile, FA could sorb onto nanoparticles and altered their surface properties. Due to the strong π-electron donor capacity of oxygen-containing functional groups on the surface of CNT and the formation of strong negatively-charge-assisted hydrogen bonds, the sorption capacity of CNT for OFL was higher than that of nano-MONT. When FA was present, the maximum sorption amount of OFL onto nano-MONT and CNT decreased by 7.4% and 24.8%, respectively. The main reason was that the complexation of FA and OFL could reduce the amount of free OFL in the solution, and the sorption affinity of FA-bound OFL onto two nanoparticles was reduced by up to 29% and 75%, respectively.</p> Conclusions <p>The presence of FA affected OFL sorption to nanoparticles. CNT sorbed more OFL than nano-MONT. FA and OFL exhibited strong binding affinity in solution and the complexation of FA-OFL reduced the sorption of OFL.</p> Graphical Abstract <p></p>