Ni-containing dealuminated Y zeolites with different SUM ratios were prepared, characterized and tested in the ethylene oligomerization reaction, performed in slurry semi-batch mode, under very mild conditions. A favourable influence of both pore accessibility and mild acidity of NiY materials on their catalytic properties was observed. With these catalysts, high activities (16-30 g of oligomers/g(cat) h) were obtained in the temperature range of 30-70 degrees C. The reaction was also highly selective, resulting almost exclusively in olefins with an even number of C-4-C-12 carbon atoms. (c) 2005 Elsevier B.V. All rights reserved.