Comprehensive experimental studies have been conducted with several varieties of commercial structured packing to ascertain geometry-imposed effects on their hydraulic behavior. At Delft, the air-water system was used for hydraulic studies, and at the the Separations Research Program (SRP) the cyclohexane/n-heptane distillation system was used to provide additional hydraulic and mass-transfer data for the same packings. These and other performance data available in the SRP database have been used to substantiate the development of a physically sound, generally applicable empirical correlation for predicting the onset of loading, where hydraulic characteristics of the packed bed change significantly. The correlation proposed in this paper covers a wide range of operating conditions and variations in structured packing geometry. It can be used with either Delft or SRP preloading models to provide an outgoing point for determining pressure drop, liquid holdup, and mass transfer in the loading region.