The ion components of the mercury argon low pressure discharge are analysed with help of a quadrupole mass spectrometer. Here we have found besides the main components Hg + and Ar + the diatomic ion-molecules Hg 2 + , Ar 2 + and ArHg + . With a discharge current of 200 mA, argon pressure 0,2 torr and mercury vapour pressure 2.8·10 -3 torr, the ion components are in the ratio [Hg + ] : [Ar + ] : [ArHg + ] : [Hg 2 + ] : [Ar 2 + ] = 1000:134:24:10:3 . The ratios i(ArHg + ) |i (Ar 2 + ), i (Ar + ) |i (Hg + ), i (Ar 2 + ) |I (Ar + ), i (Hg 2 + ) |i (Hg + ) of the effusing ion currents are measured as a function of mercury vapor pressure, discharge current and argon pressure. These measurements yield the following formation processes of ArHg + and Ar 2 + : For the products of reaction rates k and lifetimes τAr m , τAr* of the metastable and highly excited Ar-atoms, we get k ArHg + ·τ Ar m = 61·10 -16 cm 3 , k Ar 2 + · τ Ar*= 1.1·10 -16 cm 3 . So the following cross sections are found: σ ArHg + = 5.8-10 -15 cm 2 , σ Ar 2 + = 3.6-10 -15 cm .