The surface reconstructions and growth of thallium overlayers on Si(1 1 1) 7 x 7 and the adsorption geometry of phosphorus on Si(1 1 1) 7 x 7 and Ge(1 1 1) c(2 x 8) surfaces have been examined by STM and are discussed in terms of the interplay of adsorbate-induced surface strain and adsorbate-substrate bond energy effects. The adsorption of T1 layers at room temperature maintains the Si (7 x 7) reconstruction, whereas at elevated temperature, a pseudomorphic (1 x 1)-T1 overlayer is formed. The growth of T1 multilayers on these two T1 terminated monolayer surfaces, the Si-"(7 x 7)"-T1 and Si-(1 x 1)-T1, yields very different 3D T1 island shapes, thus illustrating the importance of the substrate surface potential on the growth mode. Phosphorus on Si(1 1 1)is adsorbed in a substitutional adsorption geometry in a discommensurate domain wall phase, whereas on Ge(1 1 1) it forms P trimer units in a local (root3 x root3)R30 degrees structure.