Terlipressin was chemically modified by reaction with succinimidyl propionate-monomethoxy polyethylene glycol (mPEG-SPA). To determine the PEGylated degree, the position and the optimized condition for PEGylated terlipressin, the reactions were monitored in different pH value buffers at different molar ratios by reversed-phase high performance liquid chromatography (RP-HPLC). Tryptic digestion and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used. The results showed that the amount of mono-PEG-terlipressin was higher at lower pH value and lower content of PEG. Meanwhile, the amount of di-PEG-terlipressin was higher at higher pH value and higher content of PEG under the conditions investigated. The position of PEGylated terlipressin was confirmed by tryptic digestion. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3220-3224, 2010