The rotational diffusion of polyethylene glycol coated magnetic nanoparticles in serum albumin solutions was investigated in a range spanning 0 mg mL(-1) to 200 mg mL(-1). Rotational diffusivities were determined from dynamic magnetic susceptibility measurements, which provide a non-optical means to probe rotation of nanoparticles in small volume samples. Experimental rotational diffusivities were compared to those estimated using the Stokes-Einstein relation and macroscopic measurements of the viscosity of the protein solutions. Excellent agreement was found between experimental measurements and theoretical predictions for serum albumin solutions buffered at physiological pH and for serum albumin solutions at acidic pH prepared using simple acids at physiological ionic strengths. For serum albumin solutions prepared using citrate buffer at acidic pH, we observed a discrepancy between the experimental rotational diffusivity and that predicted from the Stokes-Einstein relation. In contrast, when the pH was adjusted with a simple acid and salt at physiological ionic strength we observed agreement between the experimental rotational diffusivity and that predicted from the Stokes-Einstein relation. Because of the role of citrate ions in causing protein aggregation, we believe these observations suggest that dynamic magnetic susceptibility measurement of the rotational diffusivity of the nanoparticles is sensitive to gelation/crosslinking of proteins. (C) 2017 Elsevier Inc. All rights reserved.