The paper deals with the determination of the moment when the hydrodynamic regime in a co-current three-phase reactor changes from the gas continuous now regime to the pulsed flow regime. First, the experimental data obtained elsewhere and concerning the boundary lines between the two regimes are compiled (Table I) and compared (Fig. 1); considerable discrepancies am found in the location of these lines. Section 2 presents some mathematical criteria which basing on the known properties of the media and the packing, enable us to calculate operating parameters for which the first pulses start to appear in the reactor. Section 3 shows experimental set-up. The experiments were carried out for three spherical packing size and three basic systems: nitrogen-water, nitrogen-glycerine solutions and nitrogen-solutions of ethyl alcohol. It is found that none of the mathematical criteria proposed in the literature describes the experimental data with sufficient accuracy. The attention was focused on the model of Grosser et al. [27], which is based on the volume-averaged equations of motion for the two phases. The criterion for the change from one regime to another, proposed in [27] was derived basing on the linear stability analysis of a steady state. The analysis of the parametric sensitivity of the model of Crosser et al [27] was carried out. Such parameters were thus selected which verified enable one to fit the calculations to the experimental data They are as folows: relative permeability of two phases and the numerical coefficient in the dimensionless Leverett＇s J-function. It is necessary to verify estimated values of the parameters. This, in turn, requires the knowledge of pressure drops and the degree of saturation of the packing with gaseous and liquid phases.