The hydrogenation of p-chloronitrobenzene (p-CNB) in CO,-expanded methanol using nano-sized Ni-B as the catalyst at a p-CNB concentration of 1.5 M in a temperature range of 343 to 363 K and a pressure range of 3.54-7.09 MPa was systematically studied. The p-CNB conversions in CO2-expanded methanol were found to be significantly higher than those in methanol without containing the dissolved CO2. An appropriate ratio of CO2 partial pressure to total pressure in a range of 0-0.4 was observed to exist and to depend on temperature but to be independent of the total pressure at a fixed temperature. The volume expansion of methanol and the solubility of hydrogen in methanol were also measured in this study. The conversion enhancement was attributed to the increases in hydrogen solubility and mass transfer rate resulting from the methanol volume expansion. (C) 2006 Elsevier B.V. All rights reserved.