Bulk Keggin heteropoly acids (HPAs) H3+n[PMo12-nVnO40] (n = 0-2) and their Cs+ salts catalyse the gas-phase hydrogenation of hexanoic acid at 350 degrees C and 1 bar H-2 pressure, yielding hexanal together with 6-undecanone and C-4-C-7 hydrocarbons as the main products. Catalyst acidity (controlled by Cs substitution) has crucial effect on reaction selectivity. Partially substituted Cs salts exhibit high aldehyde selectivity (up to 76% at 70% conversion). Further increasing Cs substitution in HPAs, increases the selectivity to 6-undecanone up to 71-76% at the expense of hexanal and hydrocarbons. Initially crystalline, the post-reaction catalysts become amorphous, with their surface area significantly reduced. This shows that the as-made heteropoly compounds are catalyst precursors rather than the true catalysts. The constituent V2O5 and MoO3 oxides make more active catalysts for hexanoic acid hydrogenation than the P-Mo-V heteropoly compounds. Under reaction conditions, the bulk MoO3 reduces to MoO2, which performs steadily to give 75% hexanal selectivity at 95% conversion. The bulk V2O5, inactive itself, slowly, over 26 h, reduces to the active V2O3 oxide, which shows excellent hydrogenation performance: >97% hexanal selectivity at 80-83% conversion over 72 h on stream. (C) 2008 Elsevier B.V. All rights reserved.