Alkylation of isobutane with ethylene was investigated over a gamma-Al2O3-Cl catalyst at temperatures below 380 K. Ethylene was fully converted over a period of 5-20 h, then conversion steadily decreased. The only products were alkanes, mainly C-6 and C-8. The composition of the alkylate varied with the isobutane/ethylene ratio and with time-on-stream. Among the C-6, 2,3-dimethylbutane became predominant (> 95%) after a few hours. The composition of octanes appeared less selective. The kinetic distributions of isomers are interpreted by a new mechanism based on cyclopropane-carbonium ions. Upon catalyst ageing, alkane isomerization decays faster than alkylation, oligomerization being least affected.