The influence of liquid viscosity on liquid hold-up in structured packings under co-current gas-liquid downward flow operation has been investigated for liquid viscosity from I to 20 cP. The liquid hold-up has been determined on a 400 mm internal diameter column by gamma tomographic cross-sectional measurements. An important influence of the viscosity on the liquid hold-up is observed. It is shown that, the widely used model supported by Bravo et al. [J.L. Bravo, J.A. Rocha, J.R. Fair, Hydrocarbon Process. January (1985) 91] assuming I D fully established vertical liquid film flow does not agree with the experimental data. From experiments, the different assumptions used in the ID model are discussed. On the basis of these results, a new correlation is proposed, which enables to calculate the hold-up from the viscosity, the liquid flow rate and the geometry of the packing. A comparison with data of literature is done. (c) 2005 Elsevier B.V. All rights reserved.