A monolithic cocurrent downflow contactor (CDC) reactor was used for the palladium catalysed liquid phase hydro, enation of 2-butyne-1,4-diol (B) to cis-2-butene-1,4-diol (C) in the temperature range 293-333 K and in the pressure range 102-304 kPa. Its performance was compared with a slurry CDC reactor and a stirred tank reactor (STR). Apparent energy of activation (46-54.5 kJ/mol) and transport calculations indicated the reaction was surface rate controlled. A Langmuir-Hinshelwood mechanism was used to model the reaction. A good agreement was found between the rates from the model and the experimental rates. Selectivity to the product C was significantly greater for the monolith and slurry CDC (0.980-0.993) compared with the STR (0.90-0.95) at conversions of B approaching 100%. It was concluded that the monolith CDC possesses a performance comparable with the slurry CDC particularly in the selective production of C, but has the added advantage of allowing easy catalyst separation. (C) 2003 Elsevier Science B.V. All rights reserved.