The uptake of H2SO4 and CH3SO3H onto particles composed of water and sulfuric acid was studied in a laminar flow reactor at atmospheric pressure. Their first-order gas-phase loss rate coefficients were determined using a chemical ionization mass spectrometer. Relative humidity was varied from 6% to 97% at 295-297.5 K. The mass accommodation coefficient, alpha, was found to be close to unity for both species. These findings show that alpha does not limit particle growth rates resulting from H2SO4 and CH3SO3H uptake. Diffusion coefficients in N-2 for these two species are also reported and a significant dependence upon relative humidity was seen for H2SO4 but not for CH3SO3H. Last, production of small particles was observed due to the presence Of SO2 in particle chargers. Formation of these particles can be significantly reduced by adding an OH scavenger such as propane.