Conversion of heavy fossil hydrocarbons (HFH) to a variety of value-added chemicals and/or fuels can be enhanced using microwave (MW) and/or radio-frequency (RF) energy. Variations of reactants, process parameters, and reactor design can significantly influence the relative distribution of chemicals and fuels generated as the product. In one example, a system for flash microwave conversion of HFH includes a source concentrating microwave or RF energy in a reaction zone having a pressure greater than 0.9 atm, a continuous feed having HFH and a process gas passing through the reaction zone, a HFH-to-liquids catalyst contacting the HFH in at least the reaction zone, and dielectric discharges within the reaction zone. The HFH and the catalyst have a residence time in the reaction zone of less than 30 seconds. In some instances, a plasma can form in or near the reaction zone.