The reaction of FeS+ with methane is examined by guided ion beam mass spectrometry and density functional theory employing the B3LYP/6-311+G* level of theory. For the FeS+/CD4 system examined in the experiments, two major product ions, Fe+ and FeSD+, are observed along with minor channels leading to FeCD3+, FeSCD3+, and FeSCD+. All products are formed in endothermic processes. The measured thresholds for the formations of Fe+ and FeSD+ are compared with computational data as well as literature thermochemistry. In the theoretical approach, two competing reaction mechanisms for the formation of Fe+, concomitant with neutral methanethiol, are investigated and used to interpret the experimental data. The lowest-energy path involves a formal 1,2-addition of H3C-H across the Fe+-S bond to generate a CH3FeSH+ insertion intermediate. This bond activation step involves spin inversion from the sextet to the quartet surface en route to the products. The occurrence of the second conceivable pathway resulting in formation of HFeSCH3+ as an intermediate can be ruled out because of the high-energy demand associated with overcoming the insertion barrier along this pathway.