A series of Mo/C catalysts were prepared by equilibrium adsorption of (NH4)(6)Mo7O24. 4H(2)O onto a high surface area activated carbon. It was observed that the pH of the solution has a profound effect on the amount of Mo adsorbed on the carbon support. The Mo loading increases from ca. 0.2 to 14 wt% Mo as the pH of preparation decreases from 9 to 2. This behavior may be explained qualitatively in terms of electrostatic interactions between the molybdate ions and the protonated hydroxyl groups on the carbon surface. However, a detailed analysis of the results showed that other mechanisms which could include the participation of neutral hydroxyls may contribute to the adsorption process. The surface and bulk structure of the Mo/C catalysts were examined by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray diffraction(XRD). XRD and XPS intensity results indicated the presence of a highly dispersed monolayer-like Mo phase. TOF-SIMS and XPS valence band measurements showed that the catalysts contain polymeric Mo species at all loadings. This result was consistent with the predicted speciation based on the Mo solution chemistry and the point of zero charge of the catalysts.