This paper, first of a two-part series, discusses the results of a laboratory-scale study completed to establish the permeability variation trend with continued production of methane from coal-gas reservoirs. The field condition of uniaxial strain, assumed in the analytical models developed for permeability prediction, was replicated in the study. The results showed that the permeability of coal increases continuously, the rate of increase accelerating at low pressures. The primary reason for the increase appears to be the decrease in horizontal stress resulting from the sorption-induced volumetric strain, the so called "matrix shrinkage" effect. In the second part, experimental data is used to validate the commonly used permeability prediction analytical models and present a modification for one to improve its ability to predict permeability changes. (C) 2011 Elsevier Ltd. All rights reserved.