Solid phosphoric acid (SPA) catalyst is traditionally used in crude oil refineries to produce unhydrogenated motor-gasoline by propene and butene oligomerisation. SPA is also used in High-Temperature Fischer-Tropsch refineries (HTFT) to produce synthetic fuels albeit with a different emphasis. The petrol/diesel ratio of an HTFT refinery is very different from crude refining and it is often necessary to shift this ratio depending on market requirements. The influence of hydration was investigated as a means of improving diesel selectivity. This was achieved by studying SPA over a hydration range of 99-110% H3PO4, a temperature range of 140-230 C and using C-3-C-6 model and synthetic FT derived olefnic feedstocks. A direct correlation was found between the selectivity towards diesel range products and the distribution of the phosphoric acid species viz. H3PO4, H4P2O7 and H5P3O10. For various olefinic feedstocks, diesel selectivity increased with decreasing catalyst hydration with a maximum around 108% H3PO4 for propene oligomerisation. Commercial tests confirmed the increase in diesel selectivity with lowered catalyst hydration.(c) 2005 Elsevier B.V. All rights reserved.