1H NMR methodology is described for the determination and the characterization of S-adenosyl-L-methionine utilizing a 400 MHz spectrometer without the need of pure reference standards. The developed methodology is able to differentiate between the biologically-active (S)-S-diastereoisomer and the biologically-inactive (R)-S-diastereoisomer of S-adenosyl-L-methionine, assess chemical structure of these compounds, and determine the quantity of each isomer in the dietry suppliment formulation. The NMR methodology was found suitable to monitoring the stability of S-adenosyl-L-methionine in aqueous media, with the ability to detect degradation products adenine, homoserine lactone, S-pentosylmethionine and adenosine. The quantitative analysis was based on the integrals for the methyl proton of 2-methyl-2-propanol served as an internal standard at 1.24 ppm and the methine proton H'1 of the S-adenosyl-L-methionine ribose ring at 6.06 ppm. The accuracy was established by analyzing synthetic mixtures of the analyte and internal standard. Excellent agreement was verified between the assay results and the quantities of analyte in the mixture.