It is still not totally clear as to whether the skeletal isomerization of linear butenes to iso-butene in ferrierite occurs via a mono-molecular or bi-molecular process. To try and shed more light on this process, quantum chemical calculations were undertaken on both mechanisms. A large cluster model (H53O52Si35Al) has been employed here to study the bi-molecular process and these results are contrasted to the mono-molecular results previously reported by the author using the same model. The results suggest that a bi-molecular process can indeed result in the formation of iso-butene, as well as longer chained by-products as exemplified by 2,4,4-trimethylpent-2-ene. A rate determining step of 18.6 kcal/mol is found for the bi-molecular process, involving C-C bond formation between the two monomers. The barrier is also predicted to be considerably lower than that of the mono-molecular reaction (24.5 kcal/mol). Nevertheless, given that 2,4,4-trimethylpent-2-ene has a considerably lower barrier to reaction, and is more energetically favourable, iso-butene product to might not be expected to form in large quantities via this route. (C) 2012 Elsevier B.V. All rights reserved.