N-Hydroxysulfosuccinimide-linked biotins (sulfoNHS-biotins) are water soluble biotin tags commonly used to conjugate a biotin moiety to proteins by rapid N-acylation of primary amines. Unexpected O-acylation by sulfoNHSbiotin on tyrosine of Angiotensin II (Ag-II) and an acylation on third site unable to characterize were identified by LC-MS in addition to the expected N-acylation on the N-terminal of Ag-II. The N-acylation only undergoes incomplete hydrolysis in 0.1% formic acid not at pH 7.2 and 8.0, while two unexpected acylation hydrolyze at both conditions, but their hydrolysis in 0.1% formic acid was much more rapid. Dithiothreitol treatment selectively catalyzed hydrolysis of both of the unexpected acylation but not the N-acylation of Ag-II. The maximum yield of O-acylation of the AgII tyrosine was 99% at pH 7.2 and 95% at pH 8.0 as compared N-acylation of lysine when reacted with excess sulfoNHS-biotin with these yields of 94% at pH 7.2 and 96% at pH 8.0. Acylation of the third uncharacterized site of Ag-II showed maximum yield of approximately 17% at pH 7.2, but higher yield (≥ 47%) at pH 8.0 within 30 min. The unexpected O-acylation of the Ag-II tyrosine occurred within 1 min at either pH 7.2 or pH 8.0, as rapidly as the N-acylation, while the other unexpected acylation required more time to complete at pH 8.0.