Abstract With a developed HPLC technique for the separation of both (+)- and (−)- S -adenosylmethionine (AdoMet) and 1 H NMR analysis of the epimeric S-CH 3 chemical shifts, a kinetic study on the stability of (−)-AdoMet in solution to decomposition and epimerization is described. The results obtained from the effects of pH, temperature, and sulfonium counterions on the stability of AdoMet indicate that the epimerization appears to proceed through pyramidal inversion of the sulfonium pole. The optimal conditions for AdoMet to be stable in solution to decomposition and epimerization is to keep the compound at pH 3–5, containing an excess of large-size, nonnucleophilic counterions such as tosylate or sulfate.