© 1965-2012 IEEE. The effect of varying the pH on the ac magnetic response of the synthesized iron-oxide nanoparticles (FeOx NPs) embedded in chitosan hydrogel (CH) is experimentally demonstrated. By utilizing the chemical sensitivity of CH in FeOx-CH, we study the effect of changing the rheological property of the CH on the magnetic properties of FeOx NPs. Here, the FeOx-CH is synthesized via in situ precipitation of FeOx during CH gel formation. The morphology of the FeOx-CH is characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The swelling ratio (SR) and the ac susceptibility (ACS) of the FeOx-CH are also investigated as a function of pH. The XRD and TEM results confirm that the synthesized FeOx has a spinel crystal structure with a particle size of 10-20 nm, whereas SEM shows that the hydrogel has a porous structure. The SR result shows that FeOx-CH swells when the pH decreases (acidic). The ACS result shows that the frequency at the peak of the ACS imaginary part (χ peak frequency) shifts to higher frequency upon the decrease of pH. From these results, it is inferred that the swelling enhances the Brownian relaxation as suggested by the strong correlation between the SR and χ peak frequency at different pH conditions. Analytical calculations are also presented to estimate the change in effective viscosity due to the pH change.