The catalytic oxidation of aliphatic alcohols to carboxilic acids in aqueous solution was carried out in a fixed-bed upflow reactor by a Pd-Al2O3 catalyst. The conversion rate was measured at different gas phase velocities, alcohol concentrations and oxygen partial pressures, Single alcohol solutions, binary and ternary mixtures were tested to investigate the role of mass transfer and the influence of the reaction media composition on the reactor performance. A sensible increase of the conversion rate of single reactants was observed when mixtures of different alcohols were oxidised. The positive mixture effect seems to be related to the enhancement of the gas-liquid mass transfer coefficient.