Sulfuric acid can act as a catalyst of its own formation. We have carried out a computational investigation on the gas-phase formation of H2SO4 by hydrolysis of SO3 involving one and two water molecules, and also in the presence of sulfuric acid and its complexes with one and two water molecules. The hydrolysis of SO3 requires the concurrence of two water molecules, one of them acting as a catalyzer, and our results predict an important catalytic effect, ranging between 3 and 11 kcal.ma(-1) when the catalytic water molecule is substituted by a sulfuric acid molecule or one of its hydrates. In these cases, the reaction products are either bare sulfuric acid dimer or sulfuric acid dimer complexed with a water molecule. There are broad implications from these new findings. The results of the present investigation show that the catalytic effect of sulfuric acid in the SO3 hydrolysis can be important in the Earth's stratosphere, in the heterogeneous formation of sulfuric acid and in the formation of aerosols, in H2SO4 formation by aircraft engines, and also in understanding the formation Venus. of sulfuric acid in the atmosphere of