A Bi2O3-SnO2 system had been chosen to study the oxidative dehydrodimerisation and aromatisation of isobutene. Two different types of catalysts were prepared. One consisted of simple mechanical mixtures of the two oxides in different proportions of Bi:Sn from 0.1:1 to 10:1 and the other of co-precipitates in the same proportion of Bi:Sn. After calcination at 750degreesC, the co-precipitates were mainly composed of Bi2Sn2O7, Bi2O3, SnO2 while the mechanical mixtures were composed of Bi2O3 and SnO2 only. Catalytic testing was performed with all samples including the single oxides and it has been found that intermediate compositions of the two oxides were much better than the two single oxides both in activity and in selectivity to dimer (dimethyl hexadiene, DMH2=) and aromatics (p-xylene). All mixed samples behaved quite similarly with the Sn-rich samples being slightly better. Characterisation of the catalysts by H-2-TPR, XRD and EDX-HRTEM after reaction showed that all systems turned into a heterogeneous mixture of Bi2O3, SnO2 and a very small amount of Bi2Sn2O7 and that the Bi2O3 component is more reducible in the presence of SnO2. This accounted for the similarity in their catalytic behaviour as this change was very rapid, i.e. within the first 5 min of reaction. It is suggested that very small particles of Bi2Sn2O7 or simply the very close interaction between the two oxides is the cause of activity and selectivity to DMH2 and p-xylene. (C) 2002 Elsevier Science B.V. All rights reserved.