The carbonaceous residues formed during hydrogenation of 2-ethyl-2-hexenal (A) to 2-ethyl-hexanal (B) and 2-ethyl-hexanol (C) on a Ni/SiO2 catalyst has been studied by temperature-programmed desorption (TPD), temperature-programmed hydrogenation (TPH), temperature-programmed oxidation (TPO), and by extraction. TPD displayed below 200-degrees-C dimers, B and C. Between 200 and 400-degrees-C B, C, and alkenes were detected, and at higher temperatures alkenes and some benzenes. TPH showed the same compounds, except that methane was formed above 500-degrees-C. Both TPD and TPH produced CO, CO2, and H2O above 450-degrees-C formed by reaction of carbonaceous deposits or H-2 with unreduced NiO. TPO showed a large peak just below 400-degrees-C, a shoulder at 400-500-degrees-C, and in some experiments a small peak at about 150-degrees-C. The H/C ratio was 1.5-2.0. Above 200-degrees-C the H/C ratio increased with temperature during TPO. The total amount of carbon atoms on the surface was 10-20 times the number of surface nickel atoms indicating that carbon was also present on the support. The extract, after extraction of the used catalyst in methanol or methylene chloride, contained mainly 2-ethyl-hexanol and some 2-ethyl-hexanal.