The reactions of 2,3-dihydro-1,5-benzothiazepines with ethoxycarbonylcarbene undergo complex rearrangements to produce ring-opening and ring contraction products. Previously it was presumed that the different products were formed via different mechanisms depending on the kind of substituents at the 2-position of 1,5-benzothiazepines. However, on the basis of the further detailed investigation, it was found that all 1,5-benzothiazepines can undergo the same rearrangement to yield both ring-opening and ring contraction products.