The effect of Fischer-Tropsch derived oxygenates on olefin oligomerisation over a solid phosphoric acid (SPA) catalyst is mainly related to increased catalyst hydration and the conversion of strong pyrophosphoric acid (H4P2O7) Sites to weaker orthophosphoric acid (H3PO4) sites. At 180 degrees C, 3.8 MPa and WHSV of 1.5 h(-1), oxygenate conversion to hydrocarbons is 99%. The main oxygen-containing product was water, although alcohols produced by olefin hydration and ketone decomposition to carboxylic acids were also observed. With increasing catalyst hydration the oligomer yield decreased. The extent of inhibition was related to the oxygenate concentration, the nature of the oxygenates and the nature of the olefins. Leaching of phosphoric acid was negligible and not a significant factor in determining catalyst activity. It was shown that olefin oligomefisation on SPA follows a Langmuir-Hinshelwood mechanism. (c) 2007 Elsevier B.V. All rights reserved.