The application of fluidized bed combustion (FBC) technology to energy generation from sugar cane bagasse has been prevented, amongst other reasons, by the difficulties associated with fluidizing this material. This difficulty arises because bagasse is often lightweight and of unusual morphology, and manifests itself by either matting-up within the bed (if there is not enough inert diluent) or by segregating out, usually floating to the surface (if the inert bed material is incorrectly chosen). So, the potential benefits of mixing inert solid and fuel, desirable for efficient combustion, is lost. For FBC to be viable, proper mixing of inert fluidizing solids and fuels are necessary. A model is presented in order to explain the mixing and segregation behaviour of binary particles in fluidized beds. The conditions corresponding to equilibrium mixtures can be theoretically predicted from the physical properties of the system. Experimental observations in a gas fluidized bed are shown to support the theory. Fluidized binary mixtures exhibit no tendency to segregate under a particular combination of solids and fluid volume fractions. In these cases, the mixture remains stable, even in the absence of mixing forces. The application of the theory to bagasse combustion in fluidized bed is discussed.