The plasma catalytic oxidation of n-butane diluted in a helium-oxygen RF plasma jet is used to study volatile organic compound (VOC) removal to unravel plasma catalytic synergisms. The plasma conversion is tested for a stoichiometric n-butane oxygen mixture for varying plasma power and using a manganese oxide catalyst. It is shown that the interplay between plasma and catalyst is very complex. The catalyst enhances the power coupling but also serves as a sink for oxygen atoms due to surface recombination. The surface processes are dominated by reactions of radicals and excited species from the plasma. The oxidation of n-butane at the catalyst surface is slightly enhanced. In total, however, n-butane oxidation without the catalyst is more efficient than with the catalyst, which constitutes an anti-synergism.