A solution-based method has been developed for determination of the number of active centers on Pd particles responsible for alkene hydrogenation in the liquid phase. Pd/gamma-Al2O3 was placed in a liquid chromatographic (LC) column, and the CS2 uptake of the catalyst was determined by applying CS2 pulses in methanol solution until surface saturation was attained. The reliability of the LC method was confirmed by quantitative CS2 poisoning of the catalyst in the liquid-phase hydrogenation of styrene to ethylbenzene in methanol solution to give 5.5 +/- 0.3 mol CS2 per 100 mol of total Pd. With the adsorption stoichiometries of Pd/CO = 1 and Pd/H = 1, CO and H-2 adsorption calorimetry furnished DG = 36% for the gas-phase dispersion of the metal. This value is close to the D-TEM = 33% obtained from transmission electron microscopy analysis. With the assumption of Pd/CS2 = 6, gas-phase and liquid-phase dispersion measurements provide mutually consistent results; in this case, the liquid-phase dispersion of the metal is D-L = 33%. However, with the more generally accepted stoichiometry of Pd/CS2 = 2, solution-phase experiments yield D-L = 11%. This result implies that the number of exposed metal atoms determined from chemisorption experiments at the solid-gas interface is not equivalent to the number of catalytically active surface sites at the solid-liquid interface. (c) 2006 Elsevier Inc. All rights reserved.