The kinetics of the gas-phase reactions of SO3 with H2O and D2O were studied over the temperature range 250-360 K in N-2 with a laminar flow reactor coupled to a chemical ionization mass spectrometer. The SO3 loss is second order in the water concentration, is independent of pressure (20-80 Torr N-2, 300 K), and has a strong negative temperature dependence and a significant H/D isotope effect (k(H2O) approximate to 2k(D2O)). The yield of sulfuric acid is 1.0 +/- 0.5 per SO3 consumed. These observations are consistent with the rapid association of SO3 and H2O to form the adduct H2OSO3 which reacts with water to produce sulfuric acid. The first-order rate coefficients for loss of SO3 by reaction with H2O and D2O are given by k(I)(s(-1)) = (2.26 +/- 0.85) x 10(-43)T exp((6544 +/- 106)/T)[H2O](2) and (9.45 +/- 2.68) x 10(-44)T exp((6573 +/- 82)/T)[D2O](2), where T = K and [H2O, D2O] molecule cm(-3). The errors are the uncertainty at the 95% confidence level for precision only. Analysis of the temperature dependence of the SO3 loss yields an upper limit for the H2O-SO3 bond enthalpy of 13 kcal mol(-1).