Reactions of iron pentacarbonyl with several methyl cyclohexadienecarboxylates lead to tricarbonyliron complexes. The most stable of these is the ‘conjugated’ isomer (2; R = Me), which can be obtained from the others by treatment with methanolic acid or in some cases with methanolic methoxide. The latter method is the first observed example of base-catalysed isomerisation in tricarbonylcyclohexadieneiron complexes. When deuteriated reagents are used the acidic procedure leads to incorporation of one deuterium atom only, in the 6-position. irrespective of the starting material. Reactions of the acids with diborane yield the carbinols, one of which [tricarbonyl-1 -(hydroxymethyl)cyclohexa-1,3-dieneiron (8)], on treatment with acid, produces the rearranged carbonium salt (9). Hydride abstraction from the isomeric esters gives a series of carbonium salts, which react with water at various rates to produce hydroxy-complexes and dimeric ethers. Complexing of some methyl methylcyclohexa-1,4-dienecarboxylates is reported. Aspects of theory connected with hydrogen migration and hydride abstraction are discussed.