The understanding of the reaction mechanism and temporal speciation of the lithium sulfur batteries is challenged by complex polysulfide disproportionation chemistry coupled with the precipitation and dissolution of species. In this report, for the first time, we present a comprehensive method to investigate lithium sulfur electrochemistry using in situ ⁷Li NMR spectroscopy, which is capable of quantitatively capturing the evolution of the soluble and precipitated lithium-(poly)sulfides during electrochemical cycling. The method presented here to study the lithium sulfur battery system using in situ NMR spectroscopy can be readily applied in electrolyte/additives development and lithium protection research, and a foment clear understanding about the speciation process will accelerate electrolyte development. New information collected about the battery mechanism will not only advance Li-S sulfur battery research but also shed light to the lithium metal protection schemes.