Internal energy selected halomethane cations CH3Cl+, CH2Cl2+, CHCl3+, CH3F+, CH2F2+, CHClF2+, and CBrClF2+ were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH3F+, CH2F2+, and CH3Cl+, chlorine atom loss for CH2Cl2+, CHCl3+, and CHClF2+, and bromine atom loss for CBrClF2+. Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl3+, CHClF2+, and CBrClF2+ do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep-well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 +/- 0.01 eV, 12.30 +/- 0.02 eV, and 11.23 +/- 0.03 eV for CHCl3, CHClF2, and CBrClF2, respectively. The updated 0 K enthalpies of formation, Delta H-f degrees(0K)(g) for the ions CH2F+, CHF2+, CHCl2+, CCl3+, CCl2F+, and CClF2+ have been derived to be 844.4 +/- 2.1, 601.6 +/- 2.7, 890.3 +/- 2.2, 849.8 +/- 3.2, 701.2 +/- 3.3, and 552.2 +/- 3.4 kJ mol(-1), respectively. The Delta H-f degrees(0K)(g) values for the neutrals CCl4, CBrClF2, CClF3, CCl2F2, and CCl3F and have been determined to be -94.0 +/- 3.2, -446.6 +/- 2.7, -702.1 +/- 3.5, -487.8 +/- 3.4, and -285.2 +/- 3.2 kJ mol(-1) respectively.