Abstract: A process for impregnating with thermoplastic as well as thermosetting polymers in solid state fibers of great length, comprises realizing in a non-aqueous inert and volatile liquid in the absence of any other moistening, dispersing or emulsifying agent, a suspension of the powder polymer, continuously passing through said suspension the fibers fed in a single and preferential direction, separating such fibers during their passage through the suspension or during their staying time therein, into their unitary elements or filaments so as to expose maximum surface of each of them to contact with said suspension, assembling said unitary elements or filaments to maintain, therebetween and within fibers leaving the suspension, a given quantity of polymer powder, as well distributed as possible, eliminating the suspension liquid, submitting the obtained impregnated fibers to a thermal treatment to bring the polymer to the liquid or pasty state, then supplying the assembly to a forming or preforming unit in order to