The stability of the exchange biasing field, Heb has been studied for NiO/Ni66CO18Fe16 bilayers. A forced antiparallel alignment of the ferromagnetic magnetization to Heb results in a gradual decrease and even a reversal of Heb. The decrease of Heb increases with temperature and is independent of the external field and NiO layer thickness. This decrease can be interpreted by a thermally assisted relaxation process. A new effect of the relaxation process on Heb is demonstrated by using different cooling rates.