The isomerisations of 1-, 2- and 3-hexyne and of 1,2- and 2,3-hexadiene have been studied in various acid catalysts, all of which were anhydrous solutions in sulpholane. The principal catalysts were HBF₄ and HPF₆ while others used were BF₃, PF₅, PF₅/H₂O, HClO₄, HSO₃F, AsF₅, HAsF₆, AsF₅/SO₃, and H₂SO₄. The evidence for all catalysts favours a stepwise hexyne-hexadiene-hexyne pathway. In the case of the protonic acids it is proposed that vinyl cations were formed by protonation of the isomers and that the subsequent deprotonation occurred from another site in the molecule: -C≡C-CH₂- + H⁺⇌-CH=C-CH₂-⇌-CH=C=CH- + H⁺ The most probable explanation for the Lewis acid catalysed reaction is that a co-catalyst (such as water) is present so that the true catalyst is a protonic acid; however, alternative mechanisms are discussed. The isometric conjugated dienes were found to be unstable in the reaction media. The implications of this for the reaction mechanisms are discussed. Free energy changes in the isomerisations could not be determined since equilibration studies were not carried out due to irreversible side reactions. However, a kinetic analysis leads to an order of stability consistent with published data. The side reactions also prevented the calculation of accurate kinetic data and the reaction order. The addition products formed in the HBF₄ catalysed reactions were identified as 2,2- and 3,3-difluorohexane. Nmr, conductivity and solvent extraction studies provided evidence on the nature of the acid species and suggested that complex formation occurred between the isomers and the acids. It is proposed that the complex formation is responsible for the production of hexanones in the HPF₆ catalysed reactions. Hexanone production in the H₂SO₄ catalysed reaction was found to occur before water was added to quench the reaction, contrary to the mechanism proposed previously. A possible alternative mechanism is discussed.