We have thoroughly investigated the mechanism of the Thiemann-type SO 2 -mediated conversion of the phenylnitrile oxide (PhCNO) into phenyl isocyanate (PhNCO). The reaction proceeds through the five-membered ring intermediate, with reagents and products being separated from this intermediate by only a single transition state (Figures and), respectively. Thus, we confirmed a previously proposed mechanism for this and similar reactions. In addition, we have suggested better inorganic catalysts for such rearrangement, namely SO 3 and SeO 2 .