A comparative investigation was conducted on bubble formation and drop entrainment rates from oil mist filters in steady state operation, in order to establish a firm link between the two mechanisms. Rates were measured as a function of key filter operating parameters at different points of the filter surface, using wettable glass microfiber media as typical representatives of this application. Also, data for the thickness of the drainage film were obtained along the rear filter surface. For a given set of operating conditions, the bubble formation rate, the entrainment rate and the thickness of the oil film were found to increase roughly in proportion between the top and the bottom of the (vertically installed) filter. Bubble formation rates increased roughly tenfold along that path, from typically about 500-5000 bubbles per s and cm(2), while entrainment rose from about 10003000 small drops per s and cm(2). The drop size was narrowly confined to between about 1 and 2 mu m, regardless of vertical position on filter surface and operating conditions. Both rates increased systematically with oil loading rate, but were relatively independent of the airflow velocity. The average number of fragment drops per bubble was on the order of 1, independent of oil loading rate. Independent measurements of bubble fragmentation on a free oil surface also gave an average fragment-to-bubble ratio close to unity for small bubbles (<= 3 mm), but showed an increase similar to d(2) for larger bubbles. Overall, these measurements lend strong support to the contention that bubbling is the dominant mechanism of entrainment from the type of oil mist filter media investigated. 2017 Elsevier B.V. All rights reserved.