Although the Claus-reaction-based processes are widely timed in industry to recover sulfur from sour gas or crude oil, the precise mechanism of this reaction remains unclear and controversial. The aim of this study was to determine the chemical species of the Claus reaction in an organic liquid phase [using polyethylene glycol 400 (PEG) as solvent] at 125-135 degrees C. A combined UV-vis and Raman spectroscopic approach is developed to monitor the sulfur speciation. In addition, ab initio molecular-modeling techniques lead to an exhaustive mapping of the Raman spectral properties of molecules. Several intermediate sulfur-containing species have also been identified, as a function of experimental conditions. These analytical results are required as preliminary data to describe the chemistry of the Claus reaction in the organic liquid phase. These data are helpful to improve the kinetic model for the subdew point Claus tail gas treatment, used in the refining and natural gas treatment industries.