Tricoordinate hypervalent three-membered thiaheterocyclic intermediates of type 1 connect the starting zwitterionic imidazolium sulfinates 2, thiourea dioxides 3 and phosphine sulfide-S-dioxides 4 with the corresponding products of their thermal fragmentation, imidazolium thione 5, thioureas 6 and phosphine sulfides 7, on the reaction surface. The relative thermodynamic stability of these hypervalent, significantly ionic σ-bonding systems, explain their observed characteristic mode of thermally-allowed non-linear cheletropic fragmentation. © 1991, Taylor and Francis Group, LLC. All rights reserved.