The need to understand defluidization due to free surface liquid in gas-solid fluidized bed processes gave rise to a study of the effect of addition of non-volatile liquids with different viscosity and surface tension to a cold model gas-solid fluidized bed. The incremental addition of liquid to a fluidized bed of Geldart group B particles was observed, in general, to result in a transition to group A behaviour and eventually to group C behaviour. The BA and AC boundaries are shown to be associated with the balance between the interparticle liquid bridge forces and the fluid drag force. Using liquid bridge forces calculated from measurements of bridge geometry, the ratios of interparticle force to fluid drag force were found to vary between 0.6 and 1.07 for the AC boundary and between 0.02 and 0.06 for the BA transition boundary. Based on this approach, a practical diagram is developed enabling fluid bed behaviour to be predicted from a knowledge of particle, fluid and liquid properties and the quantity of liquid in the bed.