SO4/ZrO2 powders, showing different phase compositions, have been coupled with a traditional catalyst in Fischer-Tropsch synthesis (FTS) to convert synthesis gas to branched hydrocarbons. The isomerization efficiency, introduced by the doped oxide, was measured as a function both of the calcination temperature of the treated zirconia precursor and of the concentration of the H2SO4 used in the sulphation. Determinations of the actual acidity pK(a) and density of the oxide surface sites have been obtained by a revised Hammett-Bertolacini technique and the results are cross-compared with the catalytic data. XPS spectra of Zr 3d are discussed in respect to the conditions of the powders preparation and their phase composition. A relation between the isomerization efficiency and the pattern of Zr 3d XPS spectra is apparent. The results of the different characterisations are discussed with regard to one another and to literature data.