A two-phase structural model consisting of a dense phase and a dilute phase with allowance for the dense-phase expansion and the solids holdup in the dilute phase was developed and successfully, applied to the analysis of local voidage fluctuation signals. Such a model can be used to derive local-phase holdups and voidage in each phase in both bubbling/slugging/turbulent fluidized beds and circulating fluidized-bed risers without ambiguity, in defining criteria for dividing phases used by previous researchers. Such a model can also explain the existence of a maximum standard deviation at a local voidage around 0.7, and why a maximum peak is observed in the radial profiles of standard deviation of local voidage fluctuations in high-velocity fluidized-bed columns.