The Selected Ion Flow Tube technique requires a mass spectrometer to select the mass of the ion studied in a flow tube measurement. The performance of the SIFT developed and used in this study is discussed and the rate coefficients and product distributions for a number of positive ion-molecule reactions have been measured. The relative proton affinity of cyanoacetylene (HC₃N) has been determined to be 751±2 kJ mol-¹. A number of reactions of HC₃N with ions of importance to interstellar chemistry as well as the reactions of HC₃N⁺ and C₃N⁺ with H₂ have also been investigated. The relevance of the results obtained are discussed with respect to the chemistry of HC₃N in interstellar clouds. The relative proton affinities of acetonitrile (CH₃CN) and methyl isocyanide (CH₃NC) have been determined to be 788±2 and 844±2 kJ mol-¹ respectively. Two distinct species were observed for the adduction C₂H₄N⁺ produced in the reaction of CH₃⁺ and HCN, CH₃CNH⁺ (85±8%) and CH₃NCH⁺ (15±8%). The former species was confirmed to have the same structure as protonated acetonitrile and the latter a protonated methyl isocyanide structure. The two C₂H₄N⁺ isomers were distinguished in the flow tube by their different reactivity. A lower limit to the internal energy required to convert CH₃NCH⁺ to CH₃CNH⁺ was measured as 132 kJ mol-¹. Two stable isomeric forms of the C₂N⁺ ion were identified in the flow tube on the basis of their markedly different reactivity with CH₄. HC₂N⁺ appeared to have one stable structure only at flow tube pressures. The relative proton affinity of diacetylene (C₄H₂ was estimated to be 738±3 kJ mol-¹. Rate coefficients and product distributions were determined for reactions of the neutrals C₂H₂, HCN, C₆H₆ and H₂ with a number of hydrocarbon ions. Some evidence for isomeric forms of the ions C₄H₄⁺, C₆H₄⁺ and C₆H₅⁺ is also presented.