Cyclopentadienyl- and phenylethynyl-dimethylthallium have been prrepared from trimethylthallium. On the basis of i.r. and n.m.r. data, these compounds are given dimeric structures, with bridging cyclopentadienyl and phenylethynyl groups. Insertion of sulphur dioxide and carbon disulphide are discussed are discussed. Reactions with hydrogen halides give π-bonded derivatives.