This paper describes the preparation and the quantitative investigation of gas transport and equilibrium properties of porous glass membranes with pore diameters in a relative narrow range between 2.3 and 4.2 nm. The membranes structure and surface were characterized by low temperature nitrogen adsorption, thermogravimetry analysis and inverse gas chromatography. Single gas permeation of He, N-2, Ar, CO2 and C3H8 was measured in the temperature range of 293-433 K. To evaluate adsorption affinities of examined gases, volumetric adsorption measurements were applied at three different temperatures (293, 323 and 353 K). The permeability observations made for these membranes revealed that the transport is predominantly governed by Knudsen diffusion and viscous flow for gas phase transport and surface diffusion when an adsorbed phase is involved. Only a selective surface transport can enhance the limited Knudsen selectivity. Aiming to study also the influence of surface properties of the glass membranes on gas permeation and selectivity one membrane with a pore diameter of 2.3 nm was modified with hexamethyldisilazane. The procedure applied led to a hydrophobization and the creation of trimethylsilyl groups, which are able to fix nonpolar molecules to the surface and to provide larger separation factors between polar and nonpolar gases. The permeability of the modified membrane was found to be reduced but Knudsen diffusion was still dominating. In order to reach higher selectivity factors the pore diameter should be further reduced. A more detailed description of a subsequently prepared microporous glass membrane characterized by mass transport in the transition regime between Knudsen and configurational diffusion is presented in Part II of this paper. (C) 2009 Elsevier B.V. All rights reserved.