In the oligomerization of ethene, the chain-length distributions obtained over heterogeneous nickel catalysts at a low reaction temperature (ca. 110 degrees C) depend on the acidity of the silica-alumina support, the nickel loading and the olefin feed. Most significantly, ion-exchanged catalysts based on a silica-alumina of high acidity (ca. 0.7 mass% Ni), and those based on a highly-exchanged medium-acidity support (ca. 1.6% Ni), show a distinct superimposed activity for the dimerization of the intermediate butenes. This causes a desirable shift to higher hydrocarbon products, and it is possible to obtain 41 mass% C10+ oligomers, while the corresponding value predicted from Schulz-Flory statistics is only 24 mass%. No such deviation from Schulz-Flory statistics is observed in propene oligomerization.