Hydrogen in a hexapole collision cell is used with varying success in multi-collector Inductively Coupled Plasma Mass Spectrometry to reduce the plasma derived interferences 40Ar 16O + and 40Ar 16OH + that are isobaric with 56Fe + and 57Fe + respectively. The reactions of ArO + and ArOH + with H 2 in the hexapole of a multi-collector Inductively Coupled Plasma Mass Spectrometer were studied practically and theoretically to better constrain possible reduction mechanisms. Addition of H 2 into the hexapole caused the signal of ArOH + to increase (+ 30%) suggesting its formation there. Reactions in the hexapole cell become dominant over transmission at a lower r.f. setting for ArOH + than for ArO +, indicating that ArOH + reacts more efficiently within the hexapole. Increasing H 2 flow rate caused a decrease in background equivalent concentrations of both ArO + and ArOH + with a lower ArOH + decrease rate due to formation from 4ArO +: ArO + + H 2 → ArOH + + H followed by ArOH + + H 2 → Ar + H 2 O + + H Ab initio calculations show ArO + to have two low lying spin states; a quartet ( 4ArO +) and low lying excited doublet ( 2ArO +) that is likely to be metastable. Although highly exothermic (− 538 kJ mol − 1 ), reaction of 4ArO + with H 2 to form H 2O + is spin forbidden. Formation of ArOH + from ArO + is exothermic (− 26 kJ mol − 1 and − 51 kJ mol − 1 from 4ArO + and 2ArO + respectively) and spin-allowed, supporting the formation of ArOH + from ArO + in the hexapole. The reaction ArOH + + H 2 → Ar + H 2O + + H (− 39 kJ mol − 1 and − 61 kJ mol − 1 from 1ArOH + and 3ArOH + respectively) is likely the mechanism of ArOH + removal. For 1ArOH + (possibly produced from 2ArO +) there may be a kinetic barrier for removal, giving a possible further explanation to the persistence of ArOH +.