Hydrothermal Carbonization (HTC) is the thermochemical conversion of biomass in subcritical water to form an energy-enriched "hydrochar" as a renewable replacement for coal. Lignocellulosic biomass contains a variety of complex, interconnected biopolymers with very different physical and chemical structures, including hemicellulose, cellulose, lignin, and protein. These differing structures lead to different rates of decomposition during the HTC reaction. Where previous studies have attempted to elucidate these various rates through the use of individual, purified model compounds, the complexity of whole biomass makes understanding these reactions in their natural state difficult. This present study offers a first step toward gaining a more thorough knowledge of the HTC reaction by accurately quantifying the degradation of hemicellulose, cellulose, and lignin within whole biomass, and producing a simple kinetic and mechanistic model to describe this degradation. Australian saltbush was subjected to HTC at three temperatures (200, 230, 260 degrees C), and four residence times (0, 15, 30, 60 min). The resultant hydrochars were assayed for hemicellulose, cellulose, and lignin content, via HPLC, Updegraff assay, and acetyl bromide assay, respectively. The degradation of each component was measured, and the reaction order n and key Arrhenius parameters reaction rate constant k, activation energy E-a, and the pre-exponential factor A(0) were calculated. It was found that hemicellulose degraded fastest (n = 1, E-a = 61 kjmol(-1)), cellulose the slowest (n = 0.5, E-a = 127 kJmol(-1)), and only a portion of lignin reacted (n = 1, E-a = 66 kJmol(-1)), the remaining 22% being stable under HTC conditions.