First-order rate coefficients for the wall loss of H2SO4 were measured as a function of relative humidity in a high-pressure laminar flow tube in conjunction with chemical ionization mass spectrometry detection. The measurements yield a diffusion coefficient for H2SO4 vapor in N-2 at 298 K of 0.094 (+/-0.006) atm cm(2) s(-1) For relative humidities (RH) up to about 40%, the measured first-order loss rates steadily decreased as the RH was increased. The effective diffusion coefficient at 40% RH was similar to 20% less than without H2O present. The measured loss rates were less dependent on water vapor for RH between 40 and 70%. We interpret these observations as due to the addition of up to two H2O molecules to H2SO4, thus slowing the diffusion rate to the wall. The results indicate that about half the H2SO4 molecules are hydrated at similar to 8% RH and it is likely a second water molecule interacts with this species at higher RH. Calculations of the decrease in diffusivity of H2SO4 due to addition of water are consistent with the observed decreases.