We report on the study of the extraordinary Hall effect (EHE) in Co-Pd alloys, bi-layers, and multilayers. Polarity of the effect and its changes with aging and post-deposition annealing are attributed to a gradual intermixing and formation of interfacial Co-Pd alloys. In multilayers with sub-monoatomic layers of Co, the intermixing is mostly accomplished during the fabrication itself. In bi-layers and multilayers with thicker Co, the process of spontaneous interfacial alloying continues after the deposition with an effective room temperature alloying period of 100 days. As far as the EHE is concerned, the stabilized Co/Pd multilayers behave as alloys with effective concentration corresponding to the thickness ratio between the Co and Pd layers.