The high resolution zero-kinetic-energy (ZEKE) photoelectron spectrum of Ar-2 has been recorded between 116500 and 128500 cm(-1). The spectrum consists of a progression of 52 vibrational bands in the A (2) Sigma(1/2u)(+) <-- X (1) Sigma(g)(+) (X O-1(g)+ in Hund's case (c) notation) photoelectronic transition. The absolute numbering of the vibrational progression in the A <-- X transition is achieved by measuring the isotope shifts of two vibrational bands of the Ar-36(2) molecule. From the analysis of the vibrational progression the first adiabatic ionization potential of Ar-2 has been determined to be 116593.5 +/- 6.0 cm(-1) (14.4558 +/-0.0007 eV) from which a dissociation energy D-0 of 10601.2 +/-6.0 cm(-1) (1.3144 +/- 0.0007 eV) results for the A (2) Sigma(1/2u)(+) ground state of Ar-2(+). The potential curve of the ground ionic state in the vicinity of the potential minimum is adequately represented by a Morse potential with omega(e)(+) = 307.0 +/- 0.4 cm(-1) and omega(e)x(e)(+) = 2.05 +/- 0.05 cm(-1). The position of higher members of the vibrational progression with v(+) > 25 cannot be fitted accurately with a Morse potential. (C) 1997 American Institute of Physics.