Flow regimes varying from bubbling fluidization to pneumatic transport were characterized by analyzing the differential pressure fluctuations. Two kinds of particles, typifying those from group A and group B, were employed in the experiments. It is found that, under all the operating conditions and at various axial positions, the standard deviation of the differential pressure fluctuations almost coincides, if the solids holdup is the same. For the two particles employed, the dimensionless standard deviations normalized by the maximum standard deviation at U-c showed a unique relationship with the solids holdup, giving a favorable characteristic that allows the determination of the boundaries between the flow regimes of gas-solids fluidization. Based on the experimental results, we find approximate values of : epsilon(sb) > epsilon(s) > 0.35, for bubbling fluidization; 0.15 < epsilon(s) < 0.35, for turbulent fluidization; 0.05 < epsilon(s) < 0.15 for fast fluidization; and epsilon(s) < 0.05 for pneumatic transport.