The chemisorption of a two-level (at E 1 and E 2 ) adsorbate on the (001) surface of a tightbinding metal is investigated using the Green's function formalism and the phase shift technique. The adorbital density of states (DOS) ϱa (E) as well as the change in the electronic DOS Δ ϱ (E; E 1 , E 2 ) due to chemisorption are calculated for the ordered overlayers with c(2 x 2), p(2 x 1), p(2 X 2), p(4 X 1) and c(4 X 2) structures. It is assumed that the chemisorbed species sit over the twofold bridge site of the (001) surface of the model transition metal and have a π-bonding interaction with the two substrate atoms. It is shown that the electronic states of the overlayers are very sensitive to the adsorbate coverage (0), adsorbate structure and adsorbate species (one level or two level adsorbates). Furthermore, it is shown that there are marked differences in the Δ ϱ (E) curves between the chemisorption of two level adsorbates Δ ϱ (E; E 1 , E 2 ) and that of single level adsorbates Δ ϱ (E; E 1 ) + Δ ϱ (E; E 2 ) (simulating the changes in the electronic DOS during the dissociation of diatomic molecules).