Magnetic nanoparticles (MNPs) modified with aminoguanidine (AG) were synthesized and characterized with transmission electron microscopy (TEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and infra-red spectra (IR). Adsorption of acid dyes onto this new adsorbent was further studied in terms of pH, contact time and initial dye concentration. Maximum adsorption capacity reached from pH 1.3 to 2.5 for different dyes. The adsorption can be better described by pseudo-second-order kinetic model compared to pseudo-first-order kinetic model. The adsorption behavior fits quite well with the Langmuir rather than the Freundlich model. The mean energy of adsorption (E) calculated from the Dubinin-Radushkevich (D-R) isotherm model confirmed the involvement of chemical adsorption. This adsorbent could remove acid dyes with different structures rapidly with high adsorption capacity. Moreover, the adsorbent possesses good reusability. (C) 2013 Elsevier B.V. All rights reserved.