We have carried out adsorption and residual thermal desorption experiments of Indium on Si (111) 7 x 7 reconstructed surface, in the submonolayer regime, in Ultra High Vacuum (UHV) using in situ probes such as Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). The coverage information from AES and the surface symmetry from LEED is used to draw a 2D phase diagram which characterizes each observed superstructural phases. The different superstructural phases observed are Si(111)7 x 7-In, Si(111)root 3 x root 3R30 degrees-In, Si(111)4 x 1-In, Si(111)2 root 3 x 2 root 3R30 degrees-In and Si(111)root 7 x root 3-In in characteristic temperature and coverage regime. In addition to the 1/3 ML, root 3 x root 3-In phase, we observe two additional root 3 x root 3-In phases at around 0.6 and 1 ML. Our careful residual thermal desorption studies yields the Si(111)2 root 3 x 2 root 3R30 degrees-In phase which has infrequently appeared in the literature. We probe theoretically the structure of this phase according to the LEED structure and coverage measured by AES, assuming that the model for Si(111)2 root 3 x 2 root 3R30 degrees-In is very proximal to the well established Si(111)2 root 3 x 2 root 3R30 degrees-Sn phase, using ab initio calculation based on pseudopotentials and Density Functional Theory (DFT) to simulate an STM image of the system. Calculations show the differences in the atomic position and charge distribution in the Si(111)2 root 3 x 2 root 3R30 degrees-In case. (C) 2009 Elsevier B.V. All rights reserved.