The reorientation of magnetization is studied during film growth by means of ac susceptibility. The susceptibility is obtained via the magneto-optic Kerr effect. For Co/Au(111) a maximum of the susceptibility is found at 4.38±0.07 atomic layers of Co. The susceptibility peak is demonstrated to represent the spin-reorientation transition. The influence of external fields on the transition is explored. In agreement with theory shifts of the susceptibility peak are observed in bias fields.