Flexible graphite-based gaskets are used extensively in high-temperature applications as a replacement for asbestos-based gaskets. The effects of aging and temperature exposure on flexible graphite sheet gaskets were the subject of a previous work (Derenne et al., 1997, “Elevated Temperature Characterization of Flexible Graphite Sheet Materials for Bolted Flanged Joints,” Welding Research Council Bulletin, Issue 419, pp. 1–87.). In this paper, the effects of aging on flexible graphite under a confined-gasket configuration will be examined as they are yet unknown. This study outlines the performance evaluation of the elevated temperature behavior of flexible graphite-based gaskets under a confined configuration and long-term exposure employing a high-temperature aged leakage relaxation (HALR) fixture. This aged relaxation leakage adhesion test (ARLA)-like fixture retains the mechanical features of the aged tensile relaxation screening (ATRS)/high-temperature aged tensile relaxation (HATR) while enabling cold leakage rate and weight loss measurements. Four distinct semimetallic gaskets with different graphite-confined configurations, namely, corrugated metal, spiral wound, kammprofile, and double jacketed, were evaluated within a temperature range of 800–1200 °F (430–650 °C) for an exposure time of 2500 h. Graphite weight loss, gasket thickness change, leakage and tightness parameters, and creep and relaxation measurements were taken at regular intervals for each gasket style. To better assess the aging process, these critical mechanical and leakage properties were scrutinized; the influence of the degradation process, related mainly to graphite oxidation, was emphasized.