A pure and an antimony-modified VOx/TiO2 catalyst have been catalytically tested in a total pressure range of 1-17 bar and studied by in situ FTIR, in situ UV/vis and operando-EPR spectroscopy at normal pressure in the oxyhydrative scission (OHS) of 1-butene and n-butane to acetic acid (AA). While 1-butene OHS follows the sequence butene -> butoxide -> ketone -> acetate/AA with a multitude of trace side products also formed, n-butane OHS leads to AA, COx and H2O only. Adding water to the feed improves AA selectivity by favouring the hydrolysis of the ketone intermediate. Doping by Sb was found to improve AA selectivity being due, among other reasons, to deeper V reduction under steady-state conditions. Activity in n-butane OHS decreases continuously with rising total pressure while both activity and selectivity in I-butene OHS pass a maximum at 7 bar. (c) 2004 Elsevier B.V. All rights reserved.