An improved solid phosphoric acid catalyst has been developed for the oligomerization of C-3-C-5 alkenes, an important reaction for the refining of Fischer-Tropsch products. Catalyst particle strength is found to be the controlling design parameter when using a solid phosphoric acid catalyst for this application. The crushing strength is controlled by the relative amounts of the silicon ortho- and pyrophosphate phases present. The effect of the silicon ortho- to pyrophosphate ratio on the final catalyst was investigated by variation of key catalyst preparation parameters, namely: H3PO4 concentration, impregnation and calcination temperature and water addition. An improved catalyst with respect to crushing strength and hence lifetime, was prepared using an acid concentration of 115% H3PO4, an impregnation temperature of 220 degrees C and a calcination temperature of 420 degrees C. Water addition during calcination was found to be detrimental. Based on these findings, a new commercial catalyst was formulated which requires no binders. Comparative tests using commercial conditions showed a 30% increase in catalyst lifetime when compared to catalysts previously available. (c) 2006 Elsevier B.V. All rights reserved.