A vacuum gas oil has been co-fed with nitrogen at different partial pressures in a microacitivity (MAT) unit. The cracking rate versus hydrocarbon partial pressure can be fitted to a first-order kinetic rate equation. On the other hand, hydrogen transfer reactions, which follow second-order kinetics, are more affected than conversion by the decrease in hydrocarbon partial pressure, resulting in an increase in the yield of propylene and in general of LPG olefins. The decrease in conversion, due to the lowering of hydrocarbon partial pressure, could be compensated by a mechanical mixing effect introduced by the dilution gas that improves feed vaporization and mixing. It will be shown that a MAT unit can be used to show both effects separately, i.e. feed dilution and feed dispersion.