Chalcogen leaving groups are relatively rare in aryne forming reactions, but "onium" groups based on these have received increased attention in the past year in C-H deprotonation/-elimination sequences. As such there is relatively little known about the mechanism of this process, and nothing known when aryl thianthrenium salts are used. We measured the DKIE by one-pot competition experiments to glean an understanding of the kinetic relevance of the C-H deprotonation step in the formation of arynes from aryl thianthrenium and aryl(Mes)iodonium salts (Scheme 9). We synthesized a deuterated analog of 1b-TT and 1m-IMes to obtain the DKIE from aryl thianthrenium and aryl(Mes)iodonium salts, respectively (Scheme 9). In the case of 1b-TT and its deuterated analog, we observed a small DKIE of 1.4 (Scheme 9a), which is similar to the value obtained by Smith and co-workers for aryne formation from triaryloxonium ions. On the other hand, when we performed a competition experiment between 1m-IMes and its deuterated analog we observed a much larger DKIE of 4.9 when NaOt-Bu was used as base and 5.2 when LiHMDS was used (Scheme 9b). These data demonstrate that the C-H deprotonation step is more kinetically relevant in aryne formation from aryl(Mes)iodonium salts than from aryl thianthrenium salts. Scheme 9. DKIE experiments for aryne formation from aryl thianthrenium and aryl(Mes)iodonium salts.