The X+ (2)Pi <- X (1)Sigma(+) photoionizing transition of iodoacetylene (HC2I) has been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The resolution of the rotational structure of the spectra and its analysis provided information on the structure of the HC2I+ cation and the photoionization dynamics of HC2I. In the ground electronic (2)Pi state, the HC2I+ cation is found to be linear and subject to a strong spin-orbit coupling. The first adiabatic ionization energy of HC2I and the spin-orbit splitting of the X+ (2)Pi ground state of HC2I+ were determined to be E-1(HC2I)/hc = 78296.5(2) cm(-1) and Delta(nu) over tilde (SO) = 3257(1) cm(-1), respectively. The large spin-orbit interaction almost entirely masks the Renner-Teller effect, which is only detectable through the observation of the nominally forbidden transition to the first excited level (5(1)) of the HCC-I bending mode vs. The interaction of similar to 2 cm(-1) observed between the 5(1) levels of (2)Sigma(1/2) and (2)Delta(5/2) symmetry is attributed to a vibronic interaction with the B (2)Sigma(+) electronic state of HC2I+. The spin orbit energy level structure of tri- and tetra atomic molecules subject to the Renner-Teller effect and spin-orbit coupling is discussed for the two limiting cases where the spin-orbit-coupling constant is much smaller and much larger than the bending-mode frequencies.