The constitutive parameters (particulate phase effective elasticity, particulate effective viscosity, added mass coefficient and drag gradient) in an unstable liquid-fluidized bed are determined as functions of voidage and slip velocity. To identify these system parameters, first, forced instability wave properties are measured in water-fluidized beds, then an optimum parameter identification method is developed to reliably determine these system parameters based on a generic instability model. The results give some insight into the internal mechanics of liquid-fluidized beds and are compared with other models when they are available.