In this study, the dispersion stability of superparamagnetic iron oxide nanoparticles (IONPs) under high salinity was investigated. In our previous study, the stable dispersion of IONPs in 5 wt% NaCl solution was achieved under neutral and basic conditions through the functionalization of IONPs using sulfonated phenolic resin (SPR). However, under acidic conditions, the SPR shell dissociated from the IONP surface, resulting in the loss of dispersion stability. To overcome this limitation, crosslinking with polyvinyl alcohol (PVA) was implemented. The sulfonate groups in the SPR and hydroxyl groups in PVA can form a bond by removing water and consequently a crosslinked structure is formed between SPR and PVA. The stability of the dispersion was evaluated using visual observation of sedimentation and size measurements of the sample for a week. It was found that the dispersion was highly stable in brine, containing up to 10 wt % NaCl, at pH 4. The shell/core structure of the highly crosslinked SPR-PVA was verified using EDS mapping of the S elements in the SPR-IONP sample. The change in the shell structure through the crosslink reaction was characterized using thermogravimetric analysis and Fourier transform infrared spectroscopy. (c) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. This is an open access article under the CC BY-NC-ND license. (http://creativecommons. org/licenses/by-nc-nd/4.0/).