Complexation between human serum albumin (HSA) and poly(ethylene glycol) (PEG) was studied using different experimental techniques:  quasi-elastic light scattering (QELS), static light scattering (SLS), electrophoretic light scattering (ELS), dialysis, and fluorescence spectroscopy. The QELS study for aqueous HSA−PEG mixtures at different levels of pH and ionic strength (NaCl) showed the formation of a water-soluble complex, the size of which varied depending on both the ionic strength and the molecular weight of PEG but remained unaltered when the mixing ratio of PEG to HSA was varied. The study of the complexation in the presence and absence of 1 M urea as a function of pH by QELS and fluorescence spectroscopy strongly suggested that hydrogen bonding plays an important role in the complex formation. A combination of SLS and dialysis at pH 2 and at the ionic strength 0.1 demonstrated that the complexation yielded an “intrapolymer” complex in which several HSA molecules bound to a PEG chain. In addition, ELS indicated that the resulting intrapolymer complex behaves like a free draining coil during electrophoresis.