The paper presents an experimental study of coalescing filters for separating oil from compressed air in industrial systems. A test bench for measuring filter efficiency was set up which reproduces several typical operating conditions of pneumatic systems. Tests were carried out on commercial products, using filters of different sizes as well as several borosilicate cartridges of similar size and shape. Preliminary analysis of cartridges indicated significant differences in glass fiber dimensions and binder composition. Test results made it possible to compare performance achieved by the different configurations in terms of efficiency and pressure drop. Further measurements were repeated with different cartridge supply system geometries. One of the tested filter-cartridge-supply system configurations was then used to investigate system behavior while varying certain operating parameters individually: air velocity, oil concentration, filter supply pressure, and operating time. Results are presented in statistical form.