A molecular beam apparatus has been designed and constructed for the purpose of studying gas phase interactions between ions and molecules. A neutralization-reionization technique has been developed which permits the lifetimes of some short lived molecules to be estimated. In this way the existence of He₂, HeH and HeD molecules has been demonstrated but similar experiments have produced no evidence for the existence of H₃, D₃ or D₂H molecules with the same order of lifetimes. A collisional dissociation technique has been devised which permits analysis of molecular ions in beams and demonstrated the existence of He₃⁺ ions in low temperature helium discharges. / Thesis / Doctor of Philosophy (PhD)