The structural transformations of oriented PVA fibers impregnated with potassium bisulfate during their heat treatment in air to a temperature of 600°C were studied. The preliminary thermal stabilization of the impregnated fibers was conducted in air at 215°C for 1 h with maintenance of a high degree of orientation of the fibers. The melting peak of the preliminarily thermally stabilized impregnated PVA fibers is completely suppressed during their subsequent heat treatment in air, and the yields of coke residue are 48 and 43% of the thermally stabilized fiber mass at 500 and 600°C, respectively.