Data is presented for a number of ion-molecule and ion-atom reactions of likely relevance to the chemistry of interstellar clouds and planetary atmospheres. The measurements were made using a Selected Ion Flow Tube, (SIFT), operating at ambient temperatures, (300 ± 5 K), and at pressures between 0.30 - 0.35 Torr. The reactivity of two C₃H₃0+ isomers, namely protonated propynal, (HC=C-CHOH⁺), and the association product of C₂H₃⁺ with CO, (C₂H₃.CO), were characterised in laboratory experiments. These measurements bracket the proton affinity of propynal between 759 and 736 kJ mol⁻¹. The proton affinity of propadienone is similarly constrained between 896 and 868 kJ mol⁻¹. In a complementary investigation, the C₃H₃0⁺ potential surface was mapped using ab-initio calculations at the G2 level of theory. This combined experimental/theoretical study demonstrates that the condensation product between C₂H₃⁺ and CO is C₂-protonated propadienone, (H₂C=CH-C=O). Consequently, this finding indicates that propynal is not synthesised in the interstellar medium via the reaction of C2H3 + with carbon monoxide. The reactions between twenty-eight cations and molecular and atomic hydrogen were examined in a series of laboratory experiments. Most hydrocarbon cations react with H atoms by termolecular association or, alternatively, H atom transfer routes. In contrast, only unsaturated hydrocarbon cations tend to react with H₂ via H atom abstraction processes. There are fewer clear trends evident in the reactions between non-hydrocarbon ions and H atoms. If exothermic, H atom abstraction is almost always observed during reactions between non-hydrocarbon cations and H₂. Molecular and atomic hydrogen are both extremely plentiful in dense and diffuse interstellar clouds and these processes are therefore particularly relevant to the chemistry occurring within these objects. Results are presented for the reaction of approximately thirty cations with molecular and atomic nitrogen. Most ions are unreactive with N₂, however atomic nitrogen is a more reactive neutral species. Small hydrocarbon cations react with N atoms by forming C-N bonds and eliminating H or H₂, while larger CmHn⁺ species react with this neutral by forming HCN and a fragment hydrocarbon ion. In general there are no obvious patterns to the reactivity of non-hydrocarbon cations with atomic nitrogen. These processes are pertinent both to interstellar chemistry and the aeronomy of nitrogenous planetary atmospheres. The crucial reaction between H3 + and N has been measured for the first time in the laboratory and found to be rapid, (k= 4.5 x 10⁻¹⁰cm³). This result offers an alternative, (and possibly more facile), synthetic route to ammonia in interstellar clouds. Data is reported for the reaction of nineteen cations with O atoms, O₂ and NO. Small and medium sized hydrocarbon cations react with O atoms forming CmHnO⁺ ions and an H or H₂ fragment, whilst larger CmHn⁺ ions often react by C atom transfer forming CO and a smaller hydrocarbon ion. Most non-hydrocarbon cations examined were unreactive with atomic oxygen. Molecular oxygen is relatively unreactive with most ionic species, however many ions react with NO by charge transfer and/or termolecular association pathways. Nitric oxide, O₂ and O atoms are all present to varying degrees in interstellar clouds, hence the measurements presented here are applicable to the chemistry of these objects. A number of preliminary experiments between ions and atomic carbon are detailed. The techniques for generating C atoms are described. The research described in this thesis is pertinent to the many environments in which plasmas occur.