The direct synthesis of hydrogen peroxide has been achieved using only water as solvent in a semi-continuous stirred reactor system; this is a greener alternative over the traditional process using anthraquinone or the direct synthesis process using organic solvents (e.g. alcohols). The use of an inert co-solvent such as scCO(2) is advisable due to safety issues. Moreover, the use of scCO(2) presents additional advantages due to its acidification effect and the important enhancements it causes in the mass transfer coefficients for both the gas and the liquid phase when compared to N-2. The catalyst used, a commercial 5 wt.% Pd/C, was especially active and amounts as low as 15 mg per 200 mL of reaction volume produced remarkable results with turnover frequencies after 2 h (TOF120) over 2 mol(H2O2) h(-1) g(cat)(-1). A maximum productivity of 0.84 mmol(H2O2) min(-1) in our system was obtained for the optimized conditions, obtaining selectivities around 60% and yields around 20%. A maximum concentration of 4.3% (w/v) H2O2 was obtained after 8 h. An on-line monitoring Raman technique was used for H2O2 quantification. (C) 2011 Elsevier B.V. All rights reserved.