Several processing options are being proposed to accomplish near well-bore, downhole (in-situ) hydroprocessing of heavy crude oils. These processes are designed to pass crude oil over a fixed bed of catalyst, the catalyst being placed by conventional reservoir engineering methods. The presence of water or brine and the need to provide heat and reactant gases in a downhole environment impose challenges not present in conventional processing. In order for an in-situ process to be successful, there is a need to assess if hydroprocessing reactions under these unusual conditions are possible; and if so, how much upgrading can be obtained at processing conditions that can reasonably be obtained in an in-situ environment. Both continuous flow and batch reactor systems were used for assessing in-situ hydroprocessing of heavy crude oils. Processing conditions, including oil/water ratios, temperatures, pressures, and hydrogen flow-rates, were those expected to be achievable in a downhole environment. For the particular heavy Middle-Eastern crude oil studied, hydrodesulfurization follows pseudo-first order kinetics; activity is not catalyst dependent but instead appears to be limited by the reactivity of the feed. Additionally, from 20 to 30% of the sulfur contained in this particular crude oil consists of thermally labile sulfur; the amount of which removed remains constant provided a minimum processing temperature is attained. Density of the product decreases almost linearly with increasing reaction temperature. Reduction of hydrogen partial pressure results in a decrease in hydrodesulfurization, probably due to hydrogen starvation. Upgrading reactions occur in a sufficiently short contact time to allow for conventional oil-well production rates. These results indicate that in-situ hydroprocessing is feasible from reaction and catalyst standpoints.