Abstract The reactions of dibenzo[1,2]oxathiin 6-oxide (1a) and 3,4-dihydro-1,2-benzoxathiin 2-oxide (1b) with a phenolic leaving group were examined in acid and buffer solutions. The substrate 1b undergoes a ring opening in acid and at a higher pH, while 1a is stable in acid with a reverse ring closure predominating. The 18O-labeled 1a undergoes an isotope exchange in acid through a ring opening-closure. Although the ring opening of 1a and 1b is accelerated by buffer bases, owing to nucleophilic catalysis, a similar reaction of 3H-2,1-benzoxathiole 1-oxide with a benzylic alcohol leaving group is independent of the buffer concentration, or is decelerated by some amines. The results are accommodated by a mechanism involving a hypervalent addition intermediate with a varying rate-determining step.