Optimization of the biomass pretreatment process is crucial for overall process economics and directly impacts the performance of enzyme hydrolysis, fermentation and product recovery.  This study examined the impact of time and temperature during dilute acid pretreatment of corn stover on pretreatment sugar yields, hydrolysis performance, and cellulose accessibility.  At a constant sulfuric acid loading of 1% w/w, temperatures of 140-190°C and residence times from 1-5 min were utilized.  In agreement with compositional analysis for structural carbohydrates, the concentration of monomeric glucose gradually increased 6-fold across the entire time and temperature range while that of xylose increased 5-fold up to 190°C and then declined significantly due to sugar degradation.  The extent of xylan and lignin dissolution and degradation appeared to be important indicators of hydrolysis performance. There appears to be a delicate balance between enzyme inhibitor generation and cellulose accessibility. Cellulose accessibility increased steadily with pretreatment severity and reached a maximum at a critical temperature independent of residence time. Unexpectedly, the correlation between pretreatment severity and the degree of enzyme inhibition is very weak. The source and the chemical nature of those enzyme inhibitors will be studied further.