Cellulose is the most abundant biopolymer in the world, representing 1.5x1012 tonnes of available biomass annually. [1] As such, the development of new technologies for the use of cellulose as a feedstock for the production of fine chemicals [2-5] and hydrogen has attracted considerable interest in recent years. [6-10] Among these technologies, hydrothermal conversion is of increasing interest because pre-drying of raw materials is avoided and water gas transfer can be carried out in situ using platinum group metal catalysts. Additionally, the unique properties of compressed hot water can be tailored to control decomposition pathways. Upon hydrolysis to glucose, the decomposition of cellulose in hot compressed water follows the same path of decomposition of glucose, forming a large number of organic acids, aldehydes, ketones, furfurals and phenolic structures. as intermediaries. [11-13] Kabyemela et al., studied the decomposition kinetics of glucose, the monomer of cellulose, in subcritical water at short residence times and proposed basic non-catalytic pathways for its initial decomposition, as presented in Figure 1. These pathways formed. so far, the backbone of current understanding on the decomposition of cellulosic biomass in compressed hot water. Although it is known that gaseous products arise from short-chain aldehydes and acids [12], the acid(s) from which the gas is primarily produced and through which the intermediates are of great interest. Further work is needed to understand which pathways contribute to gas formation. Since each chemical intermediate has a different gasification rate, understanding which pathways most readily generate gasified intermediates is a primary task in terms of controlling cellulose gasification pathways. In this study, the effect of headspace fraction is investigated to determine whether the altered phase behavior of the solution affects decomposition pathways and how these can lead to gasification. The effects of sodium carbonate concentration and headspace fraction at 315°C are studied. Sodium carbonate concentrations were studied between 0 and 1 M in the presence of 1 wt%, 5% Pt/Al2O3 as metal catalyst. Headspace fractions between 49 and 93% of the reactor volume were studied at sodium carbonate concentrations of 0, 50, 100, and 500 mM. Finally, the relationship between headspace fraction and changes in liquid phase composition is discussed in terms of the balance between radical and ionic reaction pathways that mediate cellulose decomposition...