We have studied with variable temperature scanning tunneling microscopy (STM) experiments the local structure of substitutional Si defects of the 1/3 ML Sn/Si(lll) a surface. Empty state images show that most substitutional Si defects drift toward H-3 sites leaving an unsaturated dangling bond from the first layer Si atoms. Consequently there is a charge transfer toward the Sn first neighbors of the defect. Filled state images show defect centered perturbations having 3X3 symmetry, which locally modifies the surface into a honeycomb pattern. Low temperature (120 K) STM images show undoubtedly the increase of the decay length of such perturbation, indicating the possibility of an overall phase transition at much lower temperatures.