Aqueous transfer hydrogenation using Noyori's catalyst (eta(6)-arene)Ru(H)TsDPEN (TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine) was found to be dependent on the substrate/catalyst ratio. The reaction was suppressed when a large amount of substrate (acetophenone) was used. To relieve this substrate inhibition problem, common phase transfer catalysts were applied to transfer the formate anion from aqueous layer to organic substrate layer. Quaternary ammonium salts with long alkyl chains were shown to enhance the hydrogenation reactivities up to 5 times. The reaction conditions, including formate loading, phase transfer catalyst loading, amount of water, temperature, stirring speed and counterions, were optimized to afford the best catalytic activities. (C) 2014 Elsevier B.V. All rights reserved.