Successful and cost-effective PEGylation protocols require pure functionalized PEG reagents, which can be synthesized by simple and efficient procedures, exhibit high stability against hydrolysis, and maintain a level of reactivity with protein functional groups under mild reaction conditions. PEG-phenyl-isothiocyanate (PIT-PEG) is a new functionalized PEG having these characteristics, and has been synthesized by condensation of the bifunctional reagent 4-isothiocyanato phenyl isocyanate with monomethoxy PEG (mPEG). The data of (1)H NMR and colormetric analysis of the new PEG reagent establish that the mPEG has been quantitatively functionalized. The t 1/4 values for the hydrolysis of PIT-PEG5K in 100 mM phosphate solution at pH 6.5 and 9.2 are about 95 and 40 h, respectively. Incubation of human serum albumin (HSA, 0.5 mM) with a 10-fold molar excess of PIT-PEG (3K or 5K) at pH 6.5 and 9.2 generated PEG-HSA conjugates with average of 3.5 and 6.0 PEG chains per HSA molecule, respectively. The circular dichroism spectra of the conjugates showed that PEGylation of HSA has little influence on the secondary structure of HSA. The hexaPEGylated HSA, (TCP-PEG5K) 6-HSA, exhibited very high hydrodynamic volume, and the molecular radius of HSA increased from 3.95 to 6.57 nm on hexaPEGylation. The hexaPEGylation also increased the viscosity of 4% HSA from 1.05 to 2.10 cP, and the colloid osmotic pressure from 15.2 to 48.0 mmHg. The large increase in the hydrodynamic volume and the solution properties of (TCP-PEG5K) 6-HSA suggest that it could be a potential candidate as a plasma volume expander. PIT-PEG is a useful addition to the spectrum of functionalized PEG reagents available for surface decoration of proteins with PEG.