Magnesium-doped silica membranes were synthesized and a large increase in H-2/CO2 permselectivity is achieved as compared to undoped silica membranes. Three magnesium concentrations were studied, namely 10, 15 and 20 mol%, in order to find the optimal Mg-concentration for the highest H-2/CO2 separation performance. The physical properties of these sol-gel derived Mg-doped silica gels and membranes were characterized by dynamic light scattering, X-ray diffraction and high-resolution scanning electron microscopy. After the incorporation of magnesium into amorphous silica network, the membrane structure remained amorphous. Membrane performance was tested by single gas permeance of He, H-2, CO2, N-2 and CH4. With 20 mol% Mg doping, H-2/CO2 permselectivity values of more than 350 were achieved with a H-2 permeance of 70 x 10(-9) mol m(-2) s(-1) Pa-1. For pure silica membranes, a H-2/CO2 permselectivity of 9 was observed with a H-2 permeance of 526 x 10(-9) mol m-2 s(-1) Pa-1.