Experiments were carried out in an 82.6 mm diameter by 2 m high vertical column with air and water as the gas and liquid phases. Three different types of solid particles (1.5 mm glass beads, 4.5 mm glass beads and 1.2 mm steel shot) were employed to investigate the effects of particle size and density on the flow regime transitions. Superficial gas velocities ranged from 0.0018 to 7.7 m/s, while the superficial liquid velocity was varied from 0 to 0.4 m/s. Criteria for determining flow regime transitions, developed in air-water two-phase flow based on bubble frequency, Sauter mean bubble chord length and the time taken by a bubble to pass a given point, were successfully applied to gas-liquid-solid three-phase fluidized beds. These quantities were determined by a two-element conductivity probe located at the centre-line of the column 0.65 m above the distributor. It is shown that churn flow, bridging flow and annular flow occur at high gas velocities in three-phase fluidized beds, as in gas-liquid two-phase flow. Flow regime maps are presented for the air-water two-phase flow and for the three different three-phase systems. In addition, empirical correlations are provided for the flow regime boundaries in the three-phase systems investigated.