The mobility of H-2 in microporous amorphous silica is studied using quasi-elastic neutron scattering. At T = 90 K the self-diffusion coefficient is approximately D-s = 1.2 x 10(-8) m(2) s(-1) for low degrees of occupancy (< 20%) and decreases slightly to D-s = 0.95 x 10(-8) m(2) s(-1) for an occupancy of 31%. A rough estimate for the energy of activation yields E-m = 1.4 kJ mol(-1). Comparison of the results obtained for silica with those of zeolites indicates a pore size for silica in the range of 4-5 Angstrom. (C) 2003 Elsevier Science B.V. All rights reserved.