When 3-methylthiazolium iodide or BF4- is reacted with 1,1’-bis(diphenylphosphino)ferrocene-eta(2)-ethene- platinum (10) above 40 degrees C, a rapid displacement reaction occurs, leading to the formation of a ring-expanded adduct 11 by formal C-S insertion; a 2:1 adduct 12 is formed sequentially. Thiazole, its N-oxide, and oxazole do not show this type of reactivity under these conditions, although a related 1:1 adduct is formed by thiazole HO3SCF3, and both 1:1 and 2:1 adducts from the benzothiazole BF3 complex. With 2,3-dimethylthiazolium BF4-, the reaction is much slower and the product 14 arises from a C-S insertion reaction of opposite regiochemistry. With 3-methylbenzothiazolium BF4-, a similar sequence occurs, and the 1:1 adduct 17 and the 2:1 adduct 18 have been characterized crystallographically. In the benzothiazolium series, the 2,3-dimethyl derivative behaves similarly to the parent, giving product 19. Similar chemistry was observed initially with 2-chloro-3-methylbenzothiazolium BF4- on reaction with 10, but rearrangement occurs subsequently to give the C-Cl insertion product 21. C-S cleavage of S,N,N-trimethylthioformamidinium BF4- is observed on reaction with 10, giving the C-S insertion product 22. Electrospray MS played an important role in determining the solution structures of several of the Pt complexes described.