Mass-analyzed threshold ionization (MATI) spectra using coherent vacuum ultraviolet radiation have been obtained for t-butyl iodide, iso-butyl iodide, 2-iodobutane, and 1-iodobutane. The ionization energy to the lower spin-orbit state of t-butyl iodide ion has been determined to be 8.9984+/-0.0006 eV, while the threshold for fragmentation to t-C4H9+ and I has been estimated to be 9.1762 +/-0.0047 eV. Heat of formation of t-butyl cation, Delta H-f(0)(t-C4H9+), has been re-estimated, 733.7+/-3.3 kJ mol(-1). Peaks due to two different conformers of iso-butyl iodide ion, P-H and P-C, are clearly resolved in the MATI spectra, enabling the measurement of ionization energies to the lower spin-orbit states of respective conformers, 9.1725+/-0.0006 and 9.1972+/-0.0006 eV. Corresponding values for the upper spin-orbit states have been determined from the MATI spectra for the C4H9+ fragments generated by dissociation in the ion core of neutral as 9.7394+/-0.0024 and 9.7649+/-0.0023 eV. Only two out of three possible 2-iodobutane conformers have been observed in the MATI spectra with ionization energies to their lower spin-orbit states of 9.0883+/-0.0006 and 9.0913+/-0.0006 eV, even though conformer identification was not possible. Similarly, it is likely that four distinct peaks observed in the ionization threshold region of the MATI spectra of 1-iodobutane are the origins for the different conformers. Plausible mechanisms for the ion-core fragmentation of iodobutanes are discussed.