We present a theoretical analysis of the atomic and electronic structure of Si-substitutional defects on the alpha-Sn/Si(111)(root3 x root3) surface. We use a first-principles DFT local-orbital method and analyze Si-defects on a large Sn/Si(111) surface unit-cell, corresponding to a defect concentration as low as 3.7%. We also calculate the theoretical STM images, and compare with the experimental results. We find that a single Si-defect induces an upward displacement of its six nearest Sn-adatoms that shows up in filled state STM images as a bright ring of adatoms around the defect. We also analyze the atomic structure and STM images associated with three nearby Si-defects. These results are compared with those found for the Sn/Ge(111) surface. We conclude that the atomic distortion induced by Si (Ge) substitutional defects is very local, the differences found for these two surfaces being basically due to their different ground state geometries. (C) 2004 Elsevier B.V. All rights reserved.