Magnetization at 5 K and polar Kerr spectra in Co_xNi_1-x/Pt multilayers were studied as functions of Co content in order to better understand the difference in the behaviour of interfacial Co and Ni atoms. While the magneto-optic enhancement in the Kerr spectra is observed for the whole range of 0<x<1 the enhancement in the magnetization arising from the spin polarization of Pt occurs only for x>0.3 indicating that Pt atoms are not polarized by the Ni atoms. The x-ray photoelectron spectroscopy experiments indicate that there is charge transfer from Pt to 3d bands of Ni at the interface, leading to the loss of ferromagnetism in Ni. Therefore, non-magnetic Ni is not able to polarize Pt and induce a moment. In contrast, in the case of Co, no charge transfer is seen and Co remains ferromagnetic and induces a moment on Pt by polarization effects.