CO chemisorption on Bi-modified Ni(100) surfaces, along with the structure and growth of vapor-deposited Bi adlayers on Ni(100), was characterized by Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), energy loss spectroscopy (ELS), UV photoelectron spectroscopy (UPS), work function measurements, and high-resolution electron energy loss spectroscopy (HREELS). Bi growth on Ni(100) at 500 K proceeds via a layer-plus-island (Stranski - Krastanov) growth mode and the gradual formation of a c(2 x 2) structure near monolayer coverage. Desorption of Bi from the first monolayer on Ni(100) occurs with an activation energy E-d = 290 = 240 kJ mol(-1). Bi desorption from Bi multilayers has E-d = 200 kJ mol(-1). Adsorbed Bi changed the work function of the Ni(100) surface only slightly, indicating an initial dipole moment of only -0.5 D and thus relatively little charge transfer between Bi and Ni compared to other modifier adlayers. CO chemisorption was used to probe the reactivity of Ni(100) surfaces modified by preadsorbed Bi adlayers, denoted as Bi/Ni(100). Only a small decrease (4 kJ mol(-1)) occurs for the CO adsorption energy as determined by CO TPD. Site-blocking effects dominate over electronic (ligand) effects on the surface chemistry of CO on Bi/Ni(100). A comparison of these results to those on Bi/Pt(111), where Bi has been used as a model inert site-blocking agent, indicates that Bi modifies the electronic structure of Ni(100) even less than on Pt(111). Therefore. Bi adatoms may allow useful probing of adsorption and reaction ensemble requirements on Ni surfaces that contain modifiers as adatoms.