The objective of this study is to prepare ketoprofen (KP) - poly(ethylene glycol) (PEG) conjugates and to investigate their degradation kinetics. KP-PEG conjugates were synthesized from KP and PEG methy1ester by esterification in the presence of DCC. The KP-PEG conjugates (KPEG) were characterized by IR and $^{1}H-NMR$ spectroscopy. The hydrolysis of KPEG with time was studied using HPLC by simultaneous quantification of KP and KPEG. The hydrolysis rate constant was high at low and high pHs, and showed minimum at pH 4 and 5. As the size of KPEG increases, hydrolysis rate increased. The slope of degradation rate profile suggests that catalytic reaction seems to occur by specific acid/base catalysis. These results suggest that KPEG could be used as a prodrug for KP, which releases KP slowly in the body.