Acetylene . Ar complex in the S, state has been characterized through laser fluorescence excitation spectra in the acetylene ($) over tilde A<--($) over tilde X, 3(0)(n) (n=0-4) bands region. Two isomeric structures have been determined for the acetylene(($) over tilde A) Ar complex from rotational band analysis, even though only one structure was known to exist for the ground state acetylene(($) over tilde X) Ar. The in-plane isomer has the Ar atom situated in the molecular plane of the trans-bent acetylene, 3.77 Angstrom from the acetylene center of mass and tilted from the Il atoms. The out-of-plane isomeric structure, directly inverted from the rotational constants, has argon 3.76 Angstrom away from the acetylene center of mass and 18 degrees tilted from the Ct rotational axis. This structure is most likely due to large amplitude bending motions away from the equilibrium position which is along the C, axis, Axis switching effect in the rotational band analysis for both isomers has been examined and found to be negligible, (Formulas for calculating the three-dimensional axis switching angles are detailed in the Appendix.) Three van der Waals vibrational mode frequencies have been determined from the vibrational progressions in the spectra; the stretching fundamental of the out-of-plane isomer is 28 cm-l; the in-plane bending fundamental, and the out-of-plane be?ding first overtone of the in-plane isomer are II and 17 cm(-1), respectively. The isomeric structures have been compared with the results from a painwise-atom potential calculation with parameters transferred from the ones previously derived for C2H4, Ar potential calculations, It was found that when the set of parameters that most closely reflects the electronic density distribution in C2H2(A) orbitals was used, two potential minima mimicking the two isomeric structures were generated. This potential calculation can even qualitatively reproduce the complex spectral shift induced;by the v, mode excitation in acetylene. Combining the observed spectral shifts and previous experimental and theoretical studies of acetylene(X) Ar, we have estimated the binding energy of the out-of-plane C2H2(($) over tilde A).AT isomer to be 179 cm(-1), and that of the in-plane isomer to be 170 cm(-1).