The "wet pressure drop" of oil mist filters (i.e. the increase in differential pressure of the air flow due to loading of the filter with liquid) is presented as a function of two mechanisms by which coalesced oil is transported through the filter. These mechanisms operate in separate regions of the filter and make separate (and separately measurable) contributions to the overall wet pressure drop. This new concept, which was first formulated qualitatively in a phenomenological model by Ramps et at (2(114), leads to semi quantitative predictions regarding the dependence of pressure drop Lip and saturation S on filter operating conditions, filter properties and liquid properties. These predictions arc first formulated and then validated for a range of wettable and non wettable filter media in combination with 4 mineral oils of different viscosity. The key findings, summarized below, are consistent with the model and apply to both wettable and non wettable media. Oil transport across media interfaces (i.e. transitions between regions of different porosity and/or wettability) was associated with a relatively sharp increase in pressure drop Delta p and oil saturation S over a very thin layer of the filter (a "Delta p jump"). The magnitude of this Delta p jump was determined by the media properties. It correlated well with the respective static break-through pressures for oil or air, but did not depend on the oil viscosity and loading rate of the filter (at constant air velocity). Oil transport through channel regions of the filter (i.e. the regions connecting interfaces) was associated with a linear increase in Delta p with channel length and liquid throughput. The corresponding saturation level S was relatively flat throughout the channel region and lower than at an interface. (Both quantities are media dependent, of course.) An increase in oil viscosity mu (at constant oil throughput) led to different responses depending on filter wettability. (C) 2014 Elsevier Ltd. All rights reserved.