Abstract: A method of testing fibrous elements for their ability to effectively coalesce and separate water from an impure petroleum compound selected from the group consisting of crude oil and distillate fractions thereof. The method comprises introducing a reproducible emulsion consisting essentially of water and a substantially pure organic solvent into contact with a fibrous, coalescer element contained in a test zone. Separate streams of coalesced water and organic solvent are withdrawn from the zone. The predominant stream is passed between a source of light and a light-measuring means. The intensity of the light received by the light-measuring means is indicative of, and used as, the criterion for determining the acceptability of the coalescer element.