This paper describes a semiworks plant study performed to determine why vacuum distillation columns that operated well with trays experienced very poor performance when replaced with structured packings. The possible causes of the malfunctions ranged from poor design to poor wetting to Marangoni effects to phase inversion to liquid holdup problems. An experimental study was conducted to assess these effects and to determine the root cause of the problems. As a consequence of the study, guidelines for the application of structured packings in wet vacuum distillations were established. The study also illustrated a nonconventional flooding mechanism in packed towers that is exclusive to wet hydrocarbon systems; this phenomenon is described in the paper. Included in the paper are semiworks-scale efficiency and capacity data of structured packings operating under wet conditions.