Adsorption isotherms, which describe the coal's gas storage capacity, are important for estimating the carbon sequestration potential of coal seams. This study investigated the inter-laboratory reproducibility of carbon dioxide isotherm measurements on dry Argonne Premium Coal Samples (Pocahontas No. 3, Upper Freeport, Illinois No. 6, Wyodak-Anderson, and Beulah Zap). Four independent laboratories provided isotherm data for the five coal samples at temperatures of either 22 °C or 55 °C and pressures up to 7 MPa. The differences among the data sets in this study appeared to be rank-dependent in that the data among the laboratories agreed better for high-rank coal samples than for low-rank coal samples. A number of parameters such as sample size, equilibration time, and apparatus dimensions were examined to explain the rank effect, but no trend could be found that explained the differences. The variations among the data are attributed to different procedures for removing moisture to obtain the "dried" coal.