The saponin QS-21, derived from the bark of the Quillaja saponaria Molina tree, has shown great potential as an adjuvant with a number of vaccines. Kinetic studies carried out to establish the stability of vaccine formulations show that commercially supplied QS-21 (primarily QS-21A) is converted slowly at pH 5.5, and rapidly at higher pH, to an equilibrium mixture of two regioisomers, QS-21A and QS-21B, in a ratio of 20:1. NMR studies show that QS-21A and QS-21B differ only in the point of attachment of the fatty acyl moiety to the fucose sugar ring. The major isomer, QS-21A, has the fatty acyl portion attached at the 4-hydroxyl group whereas the minor isomer, QS-21B, has the fatty acyl portion attached at the 3-hydroxyl group. The isomerization most likely involves ionization of the 3-hydroxy group and intramolecular acyl transfer from the 4-hydroxy group. The relative stereochemistry of the triterpene and the sugar anomeric centers is also established by NMR methods.