We have investigated spin reorientation transitions (SRT) on magnetic ultra-thin films by X-ray magnetic circular dichroism (XMCD). The driving force of SRTs is the competition of different contribution of magnetic anisotropies: surface, interface, bulk, and shape anisotropies. The surface modification can induce SRT. The SRT is closely related to the change of the orbital magnetic moments that XMCD can probe. We reports our XMCD results for Cu/Ni/Cu(001), and Ag, NO/Co/Cu(1 1 17) surfaces together with magnetization curves by the magneto-optical Kerr effect.