We present a complete investigation into using concentrated aqueous ammonia under pressure in AFEX pretreatment with corn stover.  The complex relationship between ammonia loading, ammonia concentration, and moisture content of the biomass is studied, using subsequent enzymatic hydrolysis as a measure of the effectiveness of the pretreatment.  AFEX pretreatment parameters are reoptimized at the lowest ammonia loading that still maintains high sugar yields.  In addition, the role of reaction pressure is examined, with particular emphasis on maintaining ammonia in the liquid phase.  After reoptimizing AFEX pretreatment at lower ammonia loadings, a complete comparison to the optimal high ammonia loading conditions is made, including sugar yields, hydrolysis rates, microbial tolerance at high solid loadings, inhibition and degradation products, and fermentability of the resulting hydrolysate.