Bakken kerogens react rapidly when heated at temperatures of 40-180 degreesC to form carboxylic acid anhydrides and water from carboxylic acids. Differential scanning calorimetry (DSC) shows a pronounced irreversible endotherm over this temperature range, demonstrating the occurrence. of an endothermic chemical reaction. The fact that this reaction is the formation of an acid anhydride was demonstrated using Fourier transform infrared (FTIR) spectroscopy. The amount of anhydride formed can be estimated by measuring the enthalpy of the process using DSC. Approximately 20% of the anhydride is hydrolyzed when the reacted kerogen has been allowed to stand in air at room temperature for three weeks, demonstrating that water has access predominantly to the kerogen surface during this time. Exposure of the kerogen to water vapor at 150 degreesC for 48 h results in complete anhydride hydrolysis. Swelling the kerogen with 95 vol% tetrahydrofaran (THF)-5 vol% water also results in only partial hydrolysis of the anhydride; however, exposure to 50% aqueous THF results in complete anhydride hydrolysis. The extent of anhydride formation decreases as maturation increases. Anhydride formation has been observed with 13 of 14 kerogens that have been studied and is widespread. It occurs when either the isolated kerogen or the source rock are heated. The carboxylic acid groups must be adjacent to each other to enable such a fast reaction to occur in a glassy solid where diffusion is Strongly limited. This suggests the existence of molecular-level heterogeneity in kerogens.