There is currently much controversy surrounding homogeneous arene hydrogenation catalysis since it has been suggested that the active catalysts are actually heterogeneous. Since many of the proposed homogeneous catalysts operate in water under biphasic conditions we have studied the effect of pH on a mononuclear ruthenium catalyst precursor. As the pH increases, turnover frequency also increases, which corresponds to an increase in nanoparticle formation. The method could be used as a screen to help assess homogeneous versus heterogeneous catalysis. (C) 2003 Elsevier Science B.V. All rights reserved.