The stability of tetra-n-butylphosphonium hydroxide ([P-4444][OH]((aq))) solutions and their potential for wheat straw extraction are investigated. Under certain concentration ranges, aqueous [P-4444][OH]((aq)) is known to rapidly dissolve up to 20 wt% of cellulose at 25 degrees C. However, at elevated temperatures and at the high concentration ranges required for cellulose dissolution, [P-4444][OH]((aq)) irreversibly decomposes. This was determined by following the kinetics of decomposition at different temperatures and concentrations, using P-31 NMR analysis of the solutions. A lower concentration range of 40 wt% [P-4444][OH]((aq)) was observed for fractionation of wheat straw, avoiding significant decomposition of the expensive phosphonium component. Herein, the possibilities for producing cellulose-rich fractions with reduced lignin contents and hemicellulose-rich extracts are discussed. A proposal is given for a full process cycle using [P-4444][OH]((aq)), where the phosphonium salt is used in fractionation and recovered by anion metathesis as a chloride salt. Although not demonstrated in this article, the chloride salt may be converted back to the hydroxide by means of, e. g., ion exchange.