Cellulose is the most abundant biopolymer globally, equal to 1.5x1012 tonnes of biomass available annually. [1] Therefore, in recent years there has been considerable interest in the development of new technologies for the use of cellulose as a raw material for the production of fine chemicals [2-5] and hydrogen.[6-10] Among these technologies, hydrothermal conversion is of growing interest since pre-drying of feedstocks is avoided and water gas shifting can be performed in situ using platinum group metal catalysts. Additionally, the unique properties of compressed hot water can be tuned to control decomposition pathways. After hydrolysis into glucose, the decomposition of cellulose in compressed hot water is known to follow the same decomposition path as glucose, forming a large number of organic acids, aldehydes, ketones, furfurals, and phenolic structures as intermediates. [11-13] Kabyemela et al., studied the kinetics of the decomposition of glucose, the monomer of cellulose, in subcritical water with short residence times and proposed basic non-catalytic pathways for its initial decomposition, as presented in Figure 1. These pathways have formed the backbone of current knowledge on the decomposition of cellulosic biomass in compressed hot water. Although it is known that gaseous products originate from aldehydes and short-chain acids [12], from which acids/aldehydes the gas is primarily produced and through which intermediates is 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 generate gasified intermediates most easily is an important task in terms of how it controls the gasification pathways of cellulose. study, the effect of headspace fraction is investigated to determine whether altered solution phase behavior influences 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 the 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 the free radical and ion reaction pathways that mediate cellulose decomposition..