Upgrading heavy oils to produce transportation fuels and chemical feedstocks provides an opportunity for development of innovative new technologies. Supercritical water promoted upgrading (SCWU) has potential to reduce the sulfur content and increase the fuel and chemical content of heavy oil that reduces coke formation and hydrogen requirements compared to traditional refinery technologies. As a new technology, SCWU faces many technological challenges that are exacerbated by knowledge gaps in fundamental understanding. In particular, reaction pathways and rates, thermodynamic phase behavior, and catalyst selection must all be understood for design of processes based on SCWU. This talk will describe our progress in these areas, which have been undertaken in partnership with MIT and with the support of SAUDI ARAMCO. This work has shown that homogeneous reaction in SCWU follows radical pathways, with scission of carbon-sulfur bonds playing a key role. Moreover, we have shown that water participates in the decomposition of sulfide carbon-sulfur bonds directly as a reactant and as a solvent via hydrogen bonding. Catalysts can be selected to further reduce sulfur content. For production of fuels and chemical feeds, SCWU of vacuum residue at 450 °C and 90 min of reaction time in a batch reactor increases the diesel content from 9.9% to 25% and gasoline content from <1% to 15%. Chemical feeds, alkanes and 1-ring aromatics, were co-produced. Catalysts can be used to improve chemical yields. Catalyst stability in supercritical is a major issue; we have found that ZSM-5 is unusually stable at SCWU conditions when compared with other zeolites. Based on stability tests, we identified potential degradation products which might act catalytically and tested their activity to ensure that they did not contribute. Phase behavior was found to be a key factor determining activity, as catalysts used in reaction mixtures consisting of multiple fluid phases exhibited much less activity than when used under single fluid phase conditions – this highlights the importance of mass transport limitations. Reaction engineering studies revealed that the presence of water greatly reduces the formation of coke, while the presence of water reduces other rate constants by only about a factor of 2. Furthermore, the presence of water promotes coke gasification, preventing its accumulation. In fact, we find that coke is not a major catalyst de-activation mechanism in SCWU. We then studied the effects of co-feeding a model oil (dodecane) and aromatics on activity and selectivity, finding that aromatics reduce activity without impacting selectivity in the presence of water. Interestingly, the presence of water appears to enhance diffusion limitations, as activity in the presence of water is much more sensitive to branching than in the absence. Opportunities exist for tuning water content, as selectivity and activity are strong functions of water loading. In particular, small additions of water (5% relative to the oil feed) decrease coke formation by an order of magnitude and increase yields of desirable aromatic products. Lastly, we have studied the use of carbon coatings to improve the water stability of ZSM-5, finding that the carbon coating can improve stability of acid sites and the zeolite framework, with minimal effects on catalytic activity. Collectively, these works provide insight into the rates of SCWU reactions and the opportunities to improve reaction rates and tune product distribution.