SF6 has been used as a specific scavenger to investigate the beta/gamma radiolysis yield of hydrated electrons in pressurized high temperature sub- and supercritical water. SF6 is thermally stable in supercritical water, and each scavenging reaction is known to produce six fluoride ions, which can be readily measured using a fluoride-selective electrode. Problems in the application of this method are described, including buildup of acid product and chain reduction of SF6 in the presence of organic H-center dot and center dot OH radical scavengers. Ultimately we find that the combination of SF6 and phenol in neutral solution gives reliable results in supercritical water, because the protons and fluoride ions remain associated as HF molecules. The beta/gamma yields in supercritical water are similar to previous measurements using N2O scavenger.