Carboxylic acid-containing drugs are often metabolized to 1-beta-O-acyl glucuronides (AGs). These can undergo an internal chemical rearrangement, and the resulting reactive positional isomers can bind to endogenous proteins, with clear potential for adverse effects. Additionally any 1-beta-O-acyl-glucuronidated phase I metabolite of the drug can also show this propensity, and investigation of the adverse effect potential of a drug also needs to consider such metabolites. Here the transacylation of the common drug ibuprofen and two of its metabolites is investigated in vitro. 1-beta-O-Acyl (S)-ibuprofen glucuronide was isolated from human urine and also synthesized by selective acylation. Urine was also used as a source of the (R)-ibuprofen, (S)-2-hydroxyibuprofen, and (S,S)-carboxyibuprofen AGs. The degradation rates (a combination of transacylation and hydrolysis) were measured using 1H NMR spectroscopy, and the measured decrease in the 1-beta anomer over time was used to derive half-lives for the glucuronides. The biosynthetic and chemically synthesized (S)-ibuprofen AGs had half-lives of 3.68 and 3.76 h, respectively. (R)-Ibuprofen AG had a half-life of 1.79 h, a value approximately half that of the (S)-diastereoisomer, consistent with results from other 2-aryl propionic acid drug AGs. The 2-hydroxyibuprofen and carboxyibuprofen AGs gave half-lives of 5.03 and 4.80 h, considerably longer than that of either of the parent drug glucuronides. In addition, two (S)-ibuprofen glucuronides were synthesized with the glucuronide carboxyl function esterified with either ethyl or allyl groups. The (S)-ibuprofen AG ethyl ester and (S)-ibuprofen AG allyl esters were determined to have half-lives of 7.24 and 9.35 h, respectively. In order to construct useful structure-reactivity relationships, it is necessary to evaluate transacylation and hydrolysis separately, and here it is shown that the (R)- and (S)-ibuprofen AGs have different transacylation properties. The implications of these findings are discussed in terms of structure-activity relationships.