For the first time an examination has been made of the effect of microimpurities of oxygen, water and n-dibutyl sulphide, with their content in the reactants amounting to 0.001-0.05 wt.%, on the alkylation of isobutane with ethene on a zeolite catalyst. The process indices are most affected by the presence in the feedstock of water, which is probably due to its modifying action on the acid centres of the catalyst. Molecular oxygen at a level of 0.0001-0.002 wt.% was found to have no influence on the rate of the process. An increase in combined sulphur to 0.05 wt.% in the initial feedstock lowers the activity of the catalyst by a factor of similar to 5. Here the removal of impurities of oxygen, water and combined sulphur from the feedstock hardly alters the fractional composition of the alkylate.