The effect of support material pretreatment temperature, prior to adding the active phase and promoters, on Fischer-Tropsch activity and selectivity was explored. Four iron catalysts were prepared on silica-stabilized alumina (AlSi) supports pretreated at 700 degrees C, 900 degrees C, 1100 degrees C or 1200 degrees C. Addition of 5% silica to alumina made the AlSi material hydrothermally stable, which enabled the unusually high support pretreatment temperatures (>900 degrees C) to be studied. High-temperature dehydroxylation of the AlSi before impregnation greatly reduces FeO center dot Al2O3 surface spinel formation by removing most of the support-surface hydroxyl groups leading to more effectively carbided catalyst. The activity increases more than four-fold for the support calcined at elevated temperatures (1100-1200 degrees C) compared with traditional support calcination temperatures of <900 degrees C. This unique pretreatment also facilitates the formation of epsilon'-Fe2.2C rather than chi-Fe2.5C on the AlSi support, which shows an excellent correlation with catalyst productivity.