Chlorination of bis(supermesityl)ditelluride RTeTeR (R = 2,4,6-t-Bu3C6H2) with 3 equiv of sulfuryl chloride SO2Cl2 provided the intrinsically unstable supermesityltellurium(IV) trichloride RTeCl3 (1) as bright yellow crystals. Severe repulsion between the equatorial Cl atom and one tert-butyl group in an ortho position in the supermesityl ligand is the reason for the extreme reactivity of 1, which is in contrast to that of all previously known monoorganotellurium trihalides. In the solid state at room temperature, (the triclinic modification of) 1 reacts slowly under HCl elimination and intramolecular Te-C bond formation to give the bicyclic 5,7-di-tert-butyl-2-hydro-3,3-dimethylbenzo[b]tellurophene-1,1-dichloride (2), which was originally obtained as a colorless amorphous solid. On one occasion, when the solid-state reaction was allowed to occur under air conditions, compound 2 and a colorless crystalline byproduct, namely, trans-supermesityltellurium hydroxide dichloride (3), had formed, from which a few crystals were hand-selected. The formation of 3 has been tentatively rationalized by a solid-state hydrolysis of a second (monoclinic?) modification present in the bulk material of 1. In diethyl ether, THF, or carbon disulfide, a redox equilibrium exists between yellow supermesityltellurium(IV) trichloride RTeCl3 (1), deep blue supermesityltellurenyl(II) chloride RTeCl (4), and chlorine gas, which can be shifted to 4 when the reaction vessel is purged with argon to remove the chlorine gas. Compound 4 was also obtained by the reaction of RTeTeR (R = 2,4,6-t-Bu3C6H2) with 1 equiv of SO2Cl2. When a solution of 1 was stored with an excess of SO2Cl2 for a prolonged amount of time, Te-C bond cleavage occurred and [(Et2OH)2Te2Cl10].2Et2O (5) was formed. Compounds 1-5 have been characterized by X-ray crystallography.