ABSTRACTMagnetoresistance (MR) measurements have been performed on two series of Co(3.2 nm)/Ru(x=0.5 to 3 nm)/Co(3.2 nm) and Co(3.2 nm)/Ru(x)/Co.92Ru.08 (3.2 nm) sandwiches prepared by ultra high vacuum evaporation onto mica substrates. At 300 K, all samples of the first series present giant magnetoresistance (GMR) whereas the samples of the second series, and with the same Ru spacer thickness, show an inverse GMR. This inverse GMR is induced by alloying the second Co layer with 8 at% Ru and can be attributed to the change of the scattering coefficients ratio (α) between the two magnetic layers. Indeed, for the first Co layer the scattering coefficient ratio a1 = p↓/↑> 1. However, for the second magnetic layer, alloying Co and Ru may induce an increase of the density of states at the Fermi level for majority spin electrons and as a consequence 0C2 = p↓/↑< 1