Flooding experiments were carried out in a semicircular pilot-scale cold model of Syncrude's two fluid cokers. This unit circulates solids downward through a stripper zone and externally back to the top of the vessel, facilitating countercurrent gas/solid contacting. The onset of flooding for FCC particles was determined by analysis of differential pressure signals, supported by visual observations. The solids circulation flux at flooding increased with decreasing superficial gas velocity, with increasing fractional open area, with increasing slot width between adjacent baffles, and with decreasing baffle top included angle. A semiempirical model gives predictions of the onset of flooding that are in good agreement with both cold-flow model results and available commercial data.