The ion/molecule reactions of O-, O2-, O2-·H2O, O3-, NO3-, NO3-·HNO3, CO3-, CO4- and NO2- with methane sulfonic acid (MSA) have been studied at 300K and 1.07mb using the flowing afterglow technique. Most of the reactions give multiple products and the corresponding product yields have been deduced from the measurements. All the ion/molecule reactions, except for the NO3-·HNO3/MSA reaction, produce the CH3SO3- anion by proton transfer. The NO3-·HNO3/MSA reaction results in the formation of CH3SO3-·HNO3 product ions. An additional association channel has been observed for the CO3- and the NO2-/MSA reaction. O2-·CH3SO3H has been identified as second reaction product of the CO4-/MSA reaction. CH2SO3- product ions are also formed by the reaction of O- and CO3- with MSA. All ion/molecule reactions proceed within the experimental error at the collision rate. Several secondary product ions have also been observed and the corresponding reactions identified. The ground state geometry of MSA has been determined by quantum chemical calculations and a value of 3.776D was found for the dipole moment and 6.71Å3 for the polarizability.