The role of humidity in the hydrogenation of 2-ethyl-9,10-anthraquinone (eAQ) and especially in the further transformation of 2-ethylanthracen-9,10-diol (eAQH(2)) primary and desirable product formed on eAQ hydrogenation was studied. Three 0.5% Pd/SiO2 catalysts differing in the content of Na2CO3 and thus in alkaline properties were used. They were obtained by successive removing of Na2CO3 from the catalyst prepared by precipitation of palladium hydroxide onto Na2CO3 pre-impregnated SiO2. Hydrogenation experiments were carried out in "standard" and subsequently in "wet" systems at 62 degrees C, atmospheric pressure of H-2 in mixture of 2-octanol-xylene (1:1) as the solvent. Humidity influences the whole hydrogenation process, fundamental reaction, the hydrogenation of eAQ to eAQH(2) (quinone-hydroquinone stage) and further transformation of eAQH2. A complicated effect of humidity is observed. Humidity has an effect on consecutive reactions transforming eAQH(2) via OXO-tautomer to intermediate product 2-ethyl-9,10-dihydro-9,10-dihydroxyanthracene (INT), precursor of 2-ethylanthrone (eAN). The influence of humidity varies depending on the content of Na2CO3 in catalysts and thus their alkaline properties. The observed effects are correlated with adsorption configuration of reagents and especially eAQH(2) and INT. Highly alkaline medium as well as humidity facilitated adsorption in the carbonyl group-bonded configuration thus enhancing the transformation of eAQH(2) by hydrogenolytic reactions and especially the formation of eAN. (c) 2006 Elsevier B.V. All rights reserved.