The solid acid-catalyzed hydrolysis of cellulose was studied under elevated temperatures and autogenous pressures using in situ ATR-IR spectroscopy. Standards of cellulose and pure reaction products, which include glucose, fructose, hydroxymethylfurfural (HMF), levulinic acid (LA), formic acid, and other compounds, were measured in water under ambient and elevated temperatures. A combination of spectroscopic and HPLC analysis revealed that the cellulose hydrolysis proceeds first through the disruption of the glycosidic linkages of cellulose to form smaller cellulose molecules, which are readily observed by their distinctive CO vibrational stretches. The continued disruption of the linkages in these oligomers eventually results in the formation and accumulation of monomeric glucose. The solid-acid catalyst accelerated the isomerization of glucose to fructose, which then rapidly reacted under hydrothermal conditions to form degradation products, which included HMF, LA, formic acid, and acetic acid. The formation of these species could be suppressed by decreasing the residence time of glucose in the reactor, reaction temperature, and contact with the metal reactor. The hydrolysis of regenerated cellulose proceeded faster and under milder conditions than microcrystalline cellulose, which resulted in increased glucose yield and selectivity.