Mild acid pretreatment was performed using aqueous acetic acid or methoxyethoxy acetic acid to remove bridging cations through oxygen functional groups and to improve the pyrolytic reactivity of three coals (Beulah-Zap, BZ; Wyodak, WY; Illinois #6, IL). The acid pretreatment removed ion-exchangeable cations such as Ca and Mg from lower ranking coals, and enhanced their swelling by pyridine, and increased their volatile yield in the hash pyrolysis. The CO/CO2 ratio produced in the flash pyrolysis from lower rank coals increased by the acid treatment. The pretreatment liberates the cation-bridging network through the oxygen functional groups of coal. The acid treatment converts the carboxylates into their free carboxylic acids, enhancing the thermal decomposition into smaller hydrocarbons and their devolatilization through the pores. Acid treatment appears to loosen the pi-pi stacking and intensify the alkyl entanglement. The slow step scan XRD confirms the more influences of such treatments on the lower rank coals. (C) 2000 Elsevier Science Ltd. All rights reserved.