A one-dimensional, fluid-dynamic description of the fluidization process, which has been successfully applied to the quantitative prediction of the stability of the state of homogeneous fluidization, is extended to two-dimensional form. Numerical solutions of the resulting equations are presented which illustrate regimes off fluidization quality observed in practice: the decay of imposed particle concentration perturbations for stable systems and the development of bubbles or less pronounced concentrations inhomogeneities, depending on the specific system properties, for unstable systems. The model is, perhaps, the simplest that is necessary to capture the essential characteristics of behavior. As is the case for the one-dimensional formulation, only independent, well-established, empirical inputs are required for closure there are no arbitrary or adjustable parameters. The model predictions are shown to relate quantitatively to the theoretical transient expansion characteristics of stable liquid-fluidized beds and to reported observations of fluidization quality in both gas- and liquid-fluidized systems.