We investigate the nature and origin of the metal-insulator transition in Sr2 Ru1-x Tix O4 as a function of increasing Ti content (x). Employing detailed core, valence, and conduction band studies with x-ray and ultraviolet photoelectron spectroscopies along with Bremsstrahlung isochromat spectroscopy, it is shown that a hard gap opens up for Ti content ≥0.2, while compositions with x<0.2 exhibit finite intensity at the Fermi energy, EF. This establishes that the metal-insulator transition in this homovalent substituted series of compounds is driven by Coulomb interaction leading to the formation of a Mott gap, in contrast to transitions driven by disorder effects or band filling. © 2006 The American Physical Society.