North American production of liquefied natural gas is at an all-time high in large part due to shale gas extraction from unconventional reserves. In producing shale gas, hydraulic fracturing in combination with horizontal drilling is often used to create a path to free the hydrocarbons embedded in the reservoir. Biocides are incorporated into the fluid during drilling and fracturing for preventing activity from both native and non-native bacteria and potential reservoir souring (biogenic generation of H2S). Certain sulfur-containing biocides have been reported to be used as part of the fracturing fluid in several reservoirs. Our research has shown that at high-temperature downhole conditions, some of these compounds can decompose and generate unwanted H2S and organosulfur compounds as byproducts. In the case of dazomet, a sulfur containing biocide, the decomposition products underwent hydrolysis under downhole conditions to produce undesirable H2S, CS2, and CH3SH. A second biocide tested, methylisothiazolinone, eliminated sulfur and generated H2S by sulfur dehydrogenation of the reaction intermediates. Our findings highlight the chemical transformation that sulfur-containing biocides could undergo under hydraulic fracturing conditions. In these circumstances H2S and organosulfur compounds can be generated.