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The release of critical ash-forming elements during the pyrolysis and combustion of corn stover has been investigated through controlled lab-scale experiments supported by multicomponent and multiphase thermodynamic equilibrium calculations. Fuel samples were treated under isothermal conditions ranging from 500 to 1150 °C, under both pyrolysis and combustion atmospheres. The volatilized material was quantified by means of mass balances based on char and ash elemental analysis, compared to a corresponding feedstock fuel analysis. Close relations between the observed K and Cl release are found, suggesting that Cl is the main facilitator for K release through sublimation of KCl, determined to begin as the reaction temperature approaches 700–800 °C. K is present in abundance relative to Cl, and the K release is found to cease as the fuel reaches complete dechlorination. In addition, around 50 wt % of the Cl is released at temperatures below 500 °C, presumably as HCl formed through ion-exchange reactions with functional groups in the organic matrix. Complete dechlorination was achieved under combustion conditions as the temperature exceeded 800 °C. Approximately 50 wt % of the feedstock S is released at temperatures below 500 °C. This low-temperature release is related to the decomposition of the organic matrix, releasing the organically associated S. Under combustion conditions, the S release increases gradually at temperatures exceeding 800 °C, eventually reaching complete desulfurization at 1150 °C. The silicate/alumina chemistry is found to play a significant role in the alkali retention. The Si-rich sample is capable of retaining all excess K not released as KCl.