Abstract Vapor species of methanesulfonic acid (MSA, CH 3 SO 2 OH) trapped in solid Ar and Ar/H 2 O matrices were investigated by FTIR spectroscopy. Intensity variations induced by matrix annealing and variations in water content indicate band assignments to monomeric and dimeric MSA species and to the 1:1 MSAH2Ocomplex.AssignmentsarediscussedinrelationtoMP2andDFTcalculationsofvibrationalfrequencies.Asinglestructureissuggestedforthemonomerwhereastwostructuresofslightlydifferentstabilizationenergiesaresuggestedforboththedimerandthe1:1MSAH 2 O complex.