While several studies have been published to optimize the oxidation-exfoliation process of modified Hummers' method to make graphite oxide (GO), relatively few studies have explored the effects of operating conditions on the final GO product or process safety concerns with the GO synthesis process. In this study, reaction calorimetry is used to determine the heat of solution and oxidation reaction for a modified Hummers' method as a function of reactor processing parameters. We find that the heat of reaction increases when graphite is soaked in sulfuric acid for an extended time compared to an oxidation process without extended soaking of graphite in acid. GO synthesized with acidsoaked parent material has more surface functional groups, and the heat of the oxidation reaction decreases with increasing stirring rate. In contrast, GO synthesized with non-acid-soaked parent material has more edge functionalized groups and the heat of reaction does not vary with stirring speed. The study shows the heats of solution and reaction are high enough to reach the reported unstable temperature of Mn2O7; however, the amount of Mn2O7 generated in a typical modified Hummers' method is dilute enough to avoid a violent reaction at 55 °C.