The leakage of gas between two interconnected gas-solid fluidized beds is investigated for 48 operating conditions in a lab-scale dual fluidized bed (DFB) consisting of a riser and a bubbling fluidized bed (BFB). Helium is used as the tracer gas with a spent FCC catalyst bed in ambient conditions. Gas leakage occurs from the riser to the BFB via the downcomer; and from the BFB to the riser via the loop-seal. Gas leakage from the riser to the BFB varied between 0.1 and 0.6%, translating to 1 and 7% of the BFB being contaminated with riser gas. This leakage is dependent on pressure difference across the downcomer and the solids circulation flux. Leakage in the downcomer is proposed to be caused by the riser gas being dragged down by the falling particles in the upper section of the downcomer. (C) 2014 Elsevier B.V. All rights reserved.