The stability of Cl-terminated Si surface at ambient conditions and its evolution with time, which have immense importance for the growth of interesting nanostructures on it, were investigated using complementary methods. Wetting of water, i.e., contact angle measurements, which provide macroscopic level information, shows transition in the nature of Cl-Si surface from weak-hydrophilic toward weak-hydrophobic with time. Electron density profiles, obtained from X-ray reflectivity (XR) measurements, suggest that such a transition is associated with the growth of less uniform oxide layer. Structures of CTAB-silica mesostructured films on as-prepared and time-evolved Cl-Si substrates, obtained from XR and grazing incidence small-angle X-ray scattering measurements, show transition from strongly attached near circular micelles to weakly attached more elliptical micelles, confirming the transition (from weak-hydrophilic toward weak-hydrophobic) in microscopic level and growth of less homogeneous oxide layer. The critical time of such a transition is about 50 h, which actually represents the stability or the critical time of Cl desorption and oxide growth of the Cl-Si surface at ambient conditions.