The effects of contact time, pH, ionic strength, foreign ions, temperature, humic substances (HSs), and the addition sequences of Co(II)/HSs on Co(II) sorption on magnetic multiwalled carbon nanotube/iron oxide composites (magnetic MWCNT/IO composites) were investigated under ambient conditions. The results indicated that Co(II) sorption on the magnetic MWCNT/IO composites increased with increasing pH. The ionic strength and foreign ion dependent of Co(II) sorption suggested that ion exchange and outer-sphere surface complexation were the main sorption mechanism at low pH, whereas the ionic strength and foreign ion independent of Co(II) sorption at high pH values indicated that inner-sphere surface complexation was predominant sorption mechanism. The Langmuir model fitted Co(II) sorption isotherms better than the Freundlich model. The thermodynamic data calculated from the temperature dependent sorption isotherms suggested that Co(II) sorption on the magnetic MWCNT/IO composites was a spontaneous and endothermic process. The removal of Co(II) by the magnetic MWCNT/IO composites was affected evidently by the presence of HSs and by the addition sequences of Co(II)/HSs. This study highlights the interactions between Co(II) and HSs to determine the mutual effects of inorganic and organic matters on the removal of metal ions in environmental pollution management. (C) 2011 Elsevier B.V. All rights reserved.