The reaction of sulfur trioxide with methanesulfonic acid has been studied by examination of the nuclear magnetic resonance (n.m.r.) and Raman spectra of solutions containing up to 0.8 stoichiometric mole fraction of SO3.At relatively low SO3 concentrations the polysulfonic acid CH3S2O6H is formed initially and reacts with excess methane sulfonic acid to give methanesulfonic anhydride.[Formula: see text]CH3S2O6H and CH3SO3H exchange protons and SO3 rapidly and a single methyl resonance peak only is observed for these species in the n.m.r. spectrum. A separate methyl resonance is observed for (CH3)2S2O5 at relatively low concentrations of SO3, but at higher concentrations the peak merges with that due to CH3SO3H and CH3S2O6H because of the rapid equilibration of (CH3)2S2O5 with H2S2O7.[Formula: see text]At higher concentrations of SO3 it is likely that the system contains polymers of the three series CH3Sn+1O3n+3H, (CH3)2Sn+2O3n+5, and H2Sn+1O3n+4 in rapid equilibrium. Free SO3 is not found in the system at concentrations less than 60 mole %.