Abstract: What is described herein is a method of improving the wet tensile strength of sized glass fiber mats characterized by forming the mat from a plurality of glass fibers and a binder composition therefor which consists essentially of a urea-formaldehyde resin and about 0.01 to 5% by weight of a surfactant which is both highly water soluble and which wets the surfaces of sized glass fibers. The preferred surfactant is an anionic surfactant such as a sodium dodecylbenzene sulfonate. The glass mats thus made retained up to 79% of their tensile strength upon being subjected to severe wet conditions.