A one-dimensional hydrodynamic model of cocurrent gas-liquid downflow through hacked-bed reactors operating in the trickle flow regime is developed on the basis of a mechanistic approach. The formulation of the two-fluid model involves the mass and momentum balance equations of each fluid averaged over the void cross-sectional area of the reactor, as well as an additional equation for the capillary pressure gradient deduced from a momentum balance analysis at the gas-liquid interface. The velocity profiles for the two fluids are assumed uniform over the whole reactor cross-section. The particle-liquid and gas-liquid interactions and the column wall friction force are evaluated from closure equations based on the assumption of an annular pattern for which the gas and liquid phases are completely separated by a smooth interface, and the packing surface as well as the column wall are totally wetted by a liquid film. The linear stability analysis of the solution of the proposed hydrodynamic model around an equilibrium steady state is applied for obtaining the condition of transition to the pulsing regime. The predictions of the. trickling-to-pulsing transition given from the present model and from the existing methods are compared with a large set bf experimental data of the literature covering a wide range of operating conditions, fluids properties and packing characteristics. The two-fluid model is found to be able to predict the influences of the various parameters such as the physical properties bf fluids, the operating conditions and the packing characteristics in qualitative agreement with the experimental trends. The accuracy of the proposed model is found to be better than the ones of the existing methods.