Thermochemical sulfate reduction (TSR) is the most important mechanism for the generation of high-concentration H2S in gas reservoirs. Sulfur speciation in sour gas is one of the key factors controlling the rate and extent of TSR in gas reservoirs. However, experimental studies on S species in sour gas are limited due to the toxicity and corrosion of S and H2S. Fused silica capillary capsules (FSCCs) are inert to S and H2S and, therefore, were employed in this study as microreactors containing the S-H2S-CH4-H2O system and its subsystems, representing the composition of sour gas. The in situ Raman spectra of each system were collected continuously during the process of heating from 20 degrees C to 250 degrees C. The results showed the following: (1) a Raman peak at 2500 cm(-1) was detected in the liquid S phase of the S-H2S-CH4-H2O system at 120. 250 degrees C, which was attributed to H2Sn. A Raman band at similar to 533 cm(-1 )was detected in the aqueous phase of the S-H2S-H2O-CH4 system at 250 degrees C and was assigned to S-3(-), suggesting that S-3(-) and H2Sn are important S species in sour gas reservoirs at elevated temperatures. (2) The Raman peak at 2500 cm(-1) disappeared at 20 degrees C, indicating that H2Sn decomposes into S and H2S. During gas extraction, the decomposition of H2Sn will cause S deposition in pipelines. (3) In addition to S-3(-), H2Sn could be the intermediate valence S species involved in the TSR reaction.