Extensive experimental work was carried out with three different gas-liquid systems (air-water, air-water with surfactant C12H25SO3Na, air-aqueous CMC solution) and three kinds of packings (phi 2.7, 4.0 and 8.0 mm glass beads) to examine the influence of various parameters on pressure drop hysteresis. Gas and liquid Bow rates, physical properties of liquid and operation modes, influence the behavior of hysteresis in the packed reactor, and liquid flow rate is the most important factor. Hysteresis is not so pronounced for columns packed with large particles and it disappears in the pulsing flow regime. The mechanism responsible for hysteretic behavior resides in the variable uniformity of gas-liquid flow in the packed section. A parallel zone model for pressure drop in the trickling flow regime was established on the basis of experimental facts and analysis of flow structure. Theoretically predicted hysteresis loop of pressure drop is in satisfactory agreement with experimental data.