Scanning tunneling microscopy has been used to investigate the initial adsorption stage of Bi on Si(1 0 0) and Ge(1 0 0) surfaces at room temperature. The most favorable position for a Bi ad-dimer on both surfaces is the B-configuration (Bi ad-dimer positioned on-top of the substrate rows with its dimer bond aligned along the substrate dimer row direction). For Si(1 0 0) the A-type dimers (dimer bond aligned perpendicular to the substrate dimer row direction) occasionally rotate back and forth to a B-configuration. The diffusion rates of B-type and A-type dimers along the substrate row and the B–A, A–B rotations on Si(1 0 0) are extracted from an analysis of many sequences of STM images. Finally, it is shown that the presence of an attractive interaction between Bi ad-dimers (irrespective of their orientation) positioned on neighboring substrate dimer rows leads to the formation of local regions with a 2×2 reconstruction.