Permeation experiments on Surface Modified Graphite (SMG) membranes showed an improved mixed gas separation factor alpha (H-2/CO2) of 9.2 (+80%) and alpha (H-2/H2O) of 10.2 (+70%) compared with the pristine graphite membranes with alpha (H-2/CO2) of 5.1 and alpha (H-2/H2O) of 6.0. The surface modification was obtained by the treatment with different silanes at 140 degrees C, for 2 h under reflux and continuous stirring in an argon atmosphere. The dried powders were pressed to discs and evaluated for the ternary feed mixture (41 vol.-% H-2/41 vol.-% CO2/18 vol.-% H2O). XPS analysis of the membrane surface revealed sp(2) and sp(3) hybridisation states, carbonyl, carboxyl groups and CFx bonds for the pristine and SMG membranes. Spatially resolved micro focused Raman spectroscopy showed that the improved separation performance correlates with a specific modification of the G and D1 modes of the pristine graphite surface for each functionalisation carried out. Therefore, the improved performances result from the specific chemical and physical interactions of H-2, CO2 and H2O with the applied defect modifications of the SMG membranes.