The aryne forming step of the net dehydrogenation involves deprotonation of an aromatic C-H bond and elimination of the "onium" leaving group. We used competition and DKIE experiments as well as DFT to analyze and compare this process for aryl thianthrenium and ar-yl(Mes)iodonium salts. Our synthetic and mechanistic analysis of the formation of aryl "onium" species revealed that aryl thianthrenation is broader in arene scope than aryl iodination (vide supra). Our experimental evidence suggests that the formation of arynes from aryl(Mes)iodonium salts is broader than from aryl thianthrenium salts. In order to gain direct evidence for the relative reactivity of aryl thianthrenium and aryl(Mes)iodonium salts as aryne precursors, we performed a competition experiment with deuterium labeled substrates (Scheme 8). 2-Fluorophenol was trideuteromethylated with DMSO-d6 to generate 1z-d3. The deuterated and non-deuterated analogs of 1z-TT and 1z-IMes were synthesized and used in competition experiments to generate and trap the corresponding aryne, ultimately leading to 3za and 3za-d3 (highlighted in pink, Scheme 8). Both scenarios of the deuterium label on the aryl thianthrenium or the aryl(Mes)iodonium salts were performed and in both cases the product 3za derived from the aryl(Mes)iodonium salt was the major product by ~ 2.5: 1 (Scheme 8). These data provide direct evidence that aryl(Mes)iodonium salts are more reactive aryne precursors than the corresponding aryl thianthrenium salts.