Hypothesis: Due to attractive magnetic forces, magnetic particles (MPs) can exhibit colloidal instability upon molecular adsorption. Thus, by comparing the dye adsorption isotherms of MPs and nonmagnetic particles of the same size, shape and functional group it should be possible to characterize the influence of magnetic attraction on MP aggregation. Experiments: For a range of particle densities, a comparative adsorption study of malachite green (MG(+)) onto magnetic and non-magnetic colloids was carried out using a combination of a separation technique coupled with UV-vis spectroscopy, optical microscopy, and polarization dependent second harmonic generation (SHG) spectroscopy. Findings: Significant MP aggregation occurs in aqueous solution due to MG(+) adsorption. This alters the adsorption isotherm and challenges the determination of the adsorption equilibrium constant, K-ads center dot. The dye-induced aggregation is directly related to the MG(+) concentration, [MG(+)]. A modified Langmuir equation, which incorporates loss of surface sites due to this aggregation, accurately describes the resulting adsorption isotherms. The K-ods of 1.1 (+/- 0.3) x 10(7) and a loss of maximum MP surface capacity of 2.8 (+/- 0.7) x 10(3) M-1 per [MG(+)] has been obtained. Additionally, SHG has been established as an effective tool to detect aggregation in nanoparticles. (C) 2016 Elsevier Inc. All rights reserved.