The recently developed protocol for the absolute determination of the various polysulfide species based by H-1 NMR was applied to monitor the amount of such ions in alkaline media produced at different conditions. The thermal stability of each individual polysulfide species was also evaluated in temperature ranges representative of a typical alkaline pulping process. Overall, the species, S-1(2-), S-2(2-), S-3(2-), S-4(2-), S-5(2-), and S-6(2-) were detected and quantified in all alkaline conditions evaluated. The concentration of sodium sulfide and the temperature were found to have a significant effect on the total amount of polysulfides detected. In summary, the di- and trisulfide anions were found to predominate over the other polysulfide species in all examined experimental conditions. Additionally, the experimental values of Ea for the thermal degradation of polysulfide species correlate with the observed stability of the individual species; i. e., S-2(2-) has the highest E-a, followed by S-3(2-).