Metal dispersions of silica-supported ruthenium catalysts determined by the standard volumetric chemisorption technique were found to be inaccurate due to irreversible spillover of hydrogen to the support. Direct evidence was obtained via in situ H-1 NMR for an irreversibly bound component of the hydrogen that migrated from the metal to the support on a time scale of tens of minutes or longer at room temperature. It was also shown in this work that hydrogen saturated the surface of the metal particles on a shorter time scale, on the order of minutes. Based on H-1 NMR, optimum conditions for the volumetric chemisorption method were determined in order to alleviate the interference of spillover with measurements of hydrogen uptake on the metal. Ruthenium dispersions obtained from the optimized volumetric chemisorption technique were more accurate and agreed well with the results of the H-1 NMR method. (C) 1995 Academic Press, Inc.