The rate coefficients of the chemi-ionization collisions between Ar 3 P 2,1,0 and Hg 1 S 0 atoms are determined by optical absorption and probe measurements in decaying plasma. Th e corresponding cross section values at room temperature are as follows: σ i (Ar 3 P 2 /Hg) = (98 ± 35) Å2 , σ i (Ar 3 P 1 /Hg) = (103 ± 36) Å2 and σ i (Ar 3 P 0 /Hg) = (192 ± 67) Å2 . These values are compared with the close-collision cross sections and the cross sections calculated by the optical model of interaction.