Supercritical　water　(SCW)　fluidized　bed　reactors　convert　biomass　to　fuels　without　pollutants　emission.　In　this　work,　experimental　studies　were　carried　out　to　investigate　voidage　distribution　in　an　SCW　fluidized　bed　by　capacitance　probes.　Quartz　sands　with　different　particle　sizes　were　fluidized　by　SCW　under　system　pressure　of　20–27　MPa　and　temperature　of　410–570　°C.　The　effect　of　operation　conditions　on　voidage　distributions　of　the　emulsion　phase　(e.g.　averaged　voidage　and　probability　density)　is　discussed.　A　predicting　correlation　between　voidage　and　superficial　velocity　in　emulsion　phase　is　proposed.　The　relative　error　of　the　correlation　is　within　±25%.　These　research　results　provide　useful　guidance　for　the　optimization　of　supercritical　water　gasification　technology.　©　2021　Chinese　Society　of　Particuology　and　Institute　of　Process　Engineering,　Chinese　Academy　of　Sciences