The free surface area of a Ni/SiO2 catalyst, which has been used in hydrogenation of 2-ethylhexenal for 1 h up to 5 days, has been determined by CO adsorption and H-2-D2 reaction. CO was adsorbed at 25-degrees-C by a step adsorption technique. The area was also correlated to the H-2-D2 exchange reaction rate. The CO adsorption and the H-2-D2 reaction gave comparable results, indicating an almost linear dependence of exchange reaction and free surface area. The ratio of free surface area to initial surface area was determined to be approximately 0.1 at 51-degrees-C, approximately 0.2 at 87-degrees-C, and approximately 0.4 at 112-degrees-C after 1 h on stream. Almost all the decrease in surface area occurs when the catalyst is first exposed to the reactants. After this first decrease there is little further decrease in surface area while the conversion continues to decrease.