A new methodology of nucleophilic substitution at three different centers of a seleniranium intermediate in reactions of 2-bromomethyl-1,3-thiaselenole with mercapto benzazoles containing various combinations of heteroatoms (N, O and S) was developed. The reactions were accompanied by rearrangement with ring extension affording a new family of 2,3-dihydro-1,4-thiaselenins functionalized with benzazoles in high yields. The latter compounds in turn underwent rearrangement with ring contraction via an intermediate seleniranium cation giving new 1,3-thiaselenole ensembles in high yields. The reaction of 2-bromomethyl-1,3-thiaselenole with 1,3-benzothiazole-2-thiol proceeded in a stereoselective manner leading to (Z,Z)-CH₂CHSCHCHSeSeCHCHSCHCH₂ as the result of nucleophilic attack at the selenium atom of the seleniranium intermediate with ring opening followed by disproportionation.