Tungstated zirconia is a robust solid acid catalyst for light alkane (C-4-C-8) isomerization. Several structural models for catalytically active sites have been proposed, but the topic remains controversial, partly because of the absence of direct structural imaging information on the various supported WO x species. High-angle annular dark-field imaging of WO3/ZrO2 catalysts in an aberration-corrected analytical electron microscope allows, for the first time, direct imaging of the various species present. Comparison of the relative distribution of these WO x species in materials showing low and high catalytic activities has allowed the deduction of the likely identity of the catalytic active site-namely, subnanometre Zr-WOx clusters. This information has subsequently been used in the design of new catalysts, in which the activity of a poor catalyst has been increased by two orders of magnitude using a synthesis procedure that deliberately increases the number density of catalytically relevant active species.