Using scanning tunneling microscopy, the formation of the submonolayer In/Si(lll) interface has been studied. The main objective of this work is the determination of the composition of the forming surface phases, Si(111)root 31 x root 31-In and Si(111)4 x I-In. As a result, the absolute In coverages and the top Si atom densities have been extracted from the measured fractions of the surface area occupied by various structural domains and quantitative analysis of Si mass transport. It has been found that the surface unit cell of the 4 x I-In reconstruction contains three In atoms and two Si atoms, while the unit cell of the root 31 x root 31 -In is built of 16-17 in atoms and 26-28 Si atoms.