An industrial 20% NiO/SiO2-Al2O3 catalyst and model systems prepared by impregnation of two differently acidic SiO2-Al2O3 supports with Ni(CP)(2) or Ni(COD)(2) have been analyzed by spectroscopic in situ studies and catalytic CSTR tests during butene oligomerization under industrially relevant conditions. Active and selective sites for linear C-8 target olefins are single Nin+ species (n = 1 and/or 2) which form during conditioning by oxidative addition of Bronsted sites to Ni-0 precursor species. This is connected with a switch from an acid-catalyzed ionic mechanism to a metal-catalyzed coordinative mechanism, which shifts the product composition from strongly branched olefins to more linear olefins. Deactivation at extended times on stream is due to reaggregation of the active Ni single sites to Ni-0 clusters. Operando EPR performed for the first time in flowing butenes at 20 bar clearly shows that such clusters are not active. Bronsted sites play a crucial role in stabilizing the Ni single sites. (C) 2009 Elsevier Inc. All rights reserved.