A variety of degradation products are produced upon pretreatment of lignocellulosic biomass under different thermochemical conditions. Production and release of degradation products results from highly complex chemistry that is poorly understood at this time. Previous work has shown that the pseudo kinetics of many degradation reactions appear to demonstrate higher activation energy levels than reactions involved in the solubilisation of biomass hemicellulose. This study sought to improve the precision of previous kinetic studies in order to gain a more clear depiction of the relative kinetics and rates of accumulation of different products of biomass degradation. Samples of poplar wood and corn stover were pretreated in dilute sulfuric acid at five different temperatures (160, 170, 180, 190, 200 ºC) for reaction times of 2, 4 and 6 minutes. Long duration experiments were also carried out at 180 ºC for up to 32 minutes. Resulting hydrolysates were then filtered, extracted with MTBE and analyzed by LC-PDA-MS/MS (liquid chromatography-photodiode array-tandem mass spectrometry). Compounds quantified include aromatic acids, aliphatic acids, aldehydes and phenolic degradation products. Results from the kinetic and time sequence studies are presented.