X-ray Photoelectron Spectroscopy was applied to study the chemical composition of the modified glass surfaces by the flux of accelerated Ar or Ar/H2 plasma in order to clarify mechanisms of liquid crystal (LC) alignment on these surfaces. It was established that the surface of prinstine glass exposed to air contains carbonaceous contamination layer consisting of about 50% of aliphatic/aromatic carbon and about 50% of C-O moieties of various types. Plasma processing results in partial ablation of this layer and modification of its chemical composition. This suggests that LCs are actually being aligned by the plasma modified carbonaceous layer rather than by a pure glass.