For a better process and property control, the effect of chemical pretreatment time on the chemistry and electrochemical performance of activated carbons derived from Miscanthus grass biomass was exam-ined. The microstructure, chemistry and active functional groups were controlled by tuning the pretreatment duration, which provided the removal of certain concentrations of hemicellulose and lignin, as well as, pore development at the initial stage. The optimal KOH pretreatment (12-18 h) resulted in interconnected pore structure, rich oxygen content (18-21 at.%), significant changes in their chemistry and functional groups, and a sheet-like morphology. A high specific capacitance up to 188 F/g and a high cycling stability of 85-91% retention (after 1000-2500 cycles) at 0.1 A/g were achieved. The optimization of the pretreatment time also resulted in high specific energy (8.0 W h/kg) and specific power (377 W/ kg) at 0.5 A/g. The micro/mesopore volume, cellulose content, C/O ratio, and surface chemistry were identified to be major contributors to the electrochemical performance as a result of enhanced electroadsorption, double layer formation, and rapid ion transport. This understanding creates a simple and cost-effective route for controlling the pore network and chemistry, as well as, the resultant performance of the porous activated carbon supercapacitor electrodes. (C) 2020 Elsevier Ltd. All rights reserved.