The nature of the main primary mechanisms involved in lignocellulosic fast pyrolysis is often assumed to be radical mechanisms. Here we demonstrate that thermal depolymerization of native hemicelluloses can undergo several primary and secondary concerted reactions leading to light oxygenates that can compete with radical mechanisms. To model these reactions at a microscopic level, we used high-level quantum calculations based on functional theory. In parallel, a set of experimental data was collected to confirm the main structural features of extracted and purified hemicelluloses and to describe chemical variations within fast pyrolysis products released from various hemicellulosic fractions at 823 K. In general, the barriers computed at 800 K for pericyclic reactions were found to be reasonably low competing with these of homolytic reactions. The critical role of hydrogen bonding and spatial arrangement on product distribution was clearly demonstrated, stabilizing effects depending greatly on temperature. We reported a useful data set of intrinsic kinetic parameters and a reaction network readily available to complete kinetic models for "primary" and "secondary" fast pyrolysis of hemicelluloses.