A process for forming a self-supporting preform layer of a homogeneous blend of relatively long glass staple length fibers and wettable fine denier staple length thermoplastic filaments wherein a neutral pH aqueous slurry of the glass fibers and filaments are formed into a mat using conventional papermaking equipment. In the preferred process the wettable thermoplastic filaments are dispersed first in the water then followed by the addition of the glass fibers. The self-supporting preform when converted into a molded composite structure results in a substantially void-free structure of individual glass fibers embedded in a thermoplastic resin matrix.