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In this work, the effects of hydrogen (H2) absorption on the magnetic properties of Co/Pd multilayers are investigated. The saturation magnetization (Ms) and perpendicular anisotropy energy density (Kp) of Co/Pd multilayers were studied when the samples were exposed to one atmosphere of hydrogen (H2) and helium (He) at room temperature. Two series of (Co tCo/Pd tPd) were grown with the Co nominal thickness, tCo fixed at 2.5 A and 4.5 A and the tPd thickness, tPd varying from 0 to 25 A. When the Pd thickness was less than 10 A, there was no change in Ms and Kp. As the thickness of Pd increased the fractional change in Ms and Kp also increased. The changes in Ms and Kp in the less interdiffused samples resulted from an electron transfer to the Pd 4 d band which led to a lower density of states at the Fermi level. Magnetization measurements thus provided clear evidence for global changes of the magnetic properties of the samples upon H2 absorption, but with this technique it is impossible to determine where the H2 absorption is most prevalent, which layers are affected, and whether the Pd layers are magnetized. The bulk probe measurements in H2 were hence complemented with polarized neutron reflectivity (PNR) measurements on two samples in the presence of magnetic fields of 0.65 T and 6T applied in the plane of the samples in air and in deuterium (D2) at room temperature. The nominal Co and Pd layer thicknesses in these two samples were 2.5 A and 21 A, respectively. The PNR nuclear scattering length density showed that although D2 absorption occurred throughout the samples, absorption in the multilayer stack was small (0.02 D per Pd atom) and thus did not expand. The PNR magnetic scattering length density revealed that the Pd layers in the multilayer stack were magnetized and that their magnetization was preferentially modified upon D2 absorption, while the magnetization of the Co layers remained constant. These results indicate that H2 or D 2 absorption decrease both the perpendicular magnetic anisotropy and total magnetization of the samples. The lack of measurable expansion during absorption indicates that these changes were caused by the modification of the electronic structure of the material.