We designed and built a flexible gas-liquid pilot plant to evaluate the performances of different packings. Dynamic methods involving transferable and non-transferable tracers were used to achieve this goal. A model taking into account axial dispersions in both phases represented the packing mass transfer performances. Mass transfer coefficient, liquid and gas phases axial dispersions and holdup were estimated by performing a time domain fitting of the model response to experimental data. This method has been applied successfully to a commercially available packing (polypropylene Ralu flow 25 from Raschig).