Absolute total electron impact ionization cross-sections have been measured and calculations performed for C1 to C4 perfluorocarbons, C1 to C5 chlorocarbons and a number of mixed halo carbons and nitriles. Measured and calculated cross-sections using several different models have been analyzed using empirically and theoretically determined functions in terms of fundamental molecular parameters. A series of bond contributions to the total electron impact ionization cross-section have been determined and shown to reproduce total cross-sections for all of the molecules investigated with a high degree of confidence. An instrument has been constructed for the measurement of pseudo-absolute partial electron impact ionization cross-sections using the NIST standard mass spectrum database as a reference. Measurements were carried out for CF4, C2F6, C3F8, C3F6, F2C=CF-CF=CF2 and F3C-C=C-CF3 Theoretical models of electron impact ionization cross-sections have been critically assessed and a correction factor has been devised and applied to the binary encounter Bethe model of electron impact ionization resulting in a significant improvement in the performance of that model such that it can now be used for molecules comprising of atoms in rows 1 to 4 of the periodic table with a high degree of confidence. Ion imaging has been applied for the first time to crossed-beam electron impact ionization in an attempt to measure angular scattering patterns and energy disposal in a one step process. The instrument and the ion imaging process are described in detail and preliminary results reported in this thesis. A feasibility study has been carried out on the use of a capacitance manometer to measure neutral molecular beam fluxes directly and recommendations made for the future development of a usable device.