Three polypropylene (PP) fibers were used as admixtures in concretes studied for combustion properties (ignition temp., combustion heat, thermogravimetric (TG) anal.) and for structure. The PP fibers did differ significantly neither in ignition temps. nor in combustion heats. The presence of air during the TG anal. resulted in substantial increasing the PP fiber mass loss. Good adhesion of the fibers to concrete was obsd. The decompn. of the fibers in air started at 220 degrees C while in N-2, at 400 degrees C.