A study was conducted to determine the mode I toughness of T800H/3900-2 graphite/epoxy under a range of environments that are typical of aircraft and spacecraft operating conditions. Double cantilever beam tests were performed on both dry and moisture-saturated specimens at temperatures of -43°C, 21°C and 98°C, and on dry specimens only at 125°C. Under certain conditions, nonlinear behavior and permanent deformations were observed. Thus, a sub-study was performed to choose a data reduction method that could be used to accurately obtain the mode I toughness under all test conditions, from which a "load based compliance calibration method" was selected. At a given moisture content, it was found that toughness increased with increasing temperature. Except for the tests at -43°C, toughness was also found to increase with increasing moisture content. However, dramatically different behaviors were evidenced by the resistance curves from the various conditions. Aided by scanning electron microscopy, these results are explained by the mechanisms of crack advance, including the amount of plasticization of the matrix, matrix viscoelasticity, and the path of the growing delamination.