A molecular complex of C2H2 and AuI has been generated and isolated in the gas phase through laser ablation of a gold surface in the presence of an expanding sample containing small percentages of C2H2 and CF3I in a buffer gas of argon. Rotational, B-0, centrifugal distortion, Delta(J) and Delta(JK), and nuclear quadrupole coupling constants, chi(aa)(Au), chi(bb)(Au) - chi(cc)(Au), chi(aa)(I), and chi(bb)(I) - are measured for three isotopologues of C2H2 center dot center dot center dot AuI through broadband rotational spectroscopy. The complex is C-2v and T-shaped with C2H2 coordinating to the gold atom via donation of electrons from the pi-orbitals of ethyne. On formation of the complex, the C equivalent to C bond of ethyne extends by 0.032(4) angstrom relative to r(C equivalent to C) in isolated ethyne when the respective r(0) geometries are compared. The geometry of ethyne distorts such that angle(*-C-H) (where * indicates the midpoint of the C equivalent to C bond) is 194.7(12)degrees in the r(0) geometry of C2H2 center dot center dot center dot AuI. Ab initio calculations at the CCSD(T)(F12*)/AVTZ level are consistent with the experimentally determined geometry and further allow calculation of the dissociation energy (De) as 136 kJ mol(-1). The chi(aa)(Au) and chi(aa)(I) nuclear quadrupole coupling constants of AuI and also the Au-I bond length change significantly on formation of the complex consistent with the strong interaction calculated to occur between C2H2 and AuI.