X-Ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy are used to investigate cobalt speciation in Co ZSM -5 catalysts containing 1.4 to 9.5 wt% cobalt. These catalysts, metal-impregnated using incipient wetness techniques with Co(NO3)2 · 6H2O, are shown to contain highly dispersed, ion-exchanged, non-reducible Co2+ species interior to the ZSM-5, and large, reducible cobalt oxide crystallites on the exterior of the ZSM-5 crystallite surface. The number of Co2+ ions located inside a unit cell of ZSM-5 and the number of pyridine molecules that coordinately bond to each of these ions are estimated by using infrared data characterizing pyridine chemisorption. The crystalline forms of cobalt on the surface of the ZSM-5 are identified and their sizes estimated after O2 calcination, H2 reduction, and CO + H2 exposure. These data are then scrutinized for correlations of chemical and physical properties of the Co ZSM -5 catalysts with conversion activity and selectivity for synthesis of hydrocarbons from CO + H2 gas.