Formation of thallium overlayers on the Si(100)2 × 1 surface has been studied using first-principles total-energy calculations. According to total-energy calculations and STM image modeling, thallium atoms should occupy the following positions on Si(100) surface: (i) valley-bridge (0.25 ML of thallium coverage); (ii) parallel Tl-Tl dimer in valley-bridge (0.5 ML); (iii) pedestal, valley-bridge and valley-bridge (0.75 ML); and (iv) pedestal and valley-bridge (1.0 ML). The simulated STM images are in good agreement with experimental data [A.A. Saranin et al. Phys. Rev. B 71, 035312 (2005)]. The possible atomic mechanism of reconstruction alternation is discussed. [DOI: 10.1380/ejssnt.2005.125]