The interaction of SO3 and H2O at low temperatures upon an inert surface has been studied with infrared spectroscopy and compared to the predictions of recent computational studies. At low temperatures and low water partial pressures, amorphous deposits of molecular H2SO4 complexed with variable amounts of H2O in a ratio of between 1:1 and 2:1 are formed. Upon annealing, this material ejects water and converts first to a 1:1 H2SO4.H2O Complex and subsequently to anhydrous H2SO4. Adding water to the amorphous molecular hydrate results in the formation of a new species, which on the basis of its thermal behavior and by comparison to theoretical predictions can be attributed to a molecular polymer with a repeat unit of (H2SO4.(H2O)(2))(n). Implications of these observations for the initial stages of the formation of sulfate aerosol in the atmosphere and their surface reactivity are discussed.