The direct decay induced ⁸²Br (or ⁸⁰Br) labelling by exposing the solid substrate molecules, such as deoxyuridine, L-tyrosine, guanosine, deoxycytodine, phenylalanine and acetic acid, to gaseous CF₃^82mBr (or CF₃^80mBr) was studied. The radiochemical yields of the brominated products are relatively small and range from 1% in the case of bromo-deoxyuridine to 11% for bromoacetic acid. The modification of this technique by adding Cl₂ gas to the reaction mixture improves the yields in several cases drastically (up to 80% for bromo-guanosine and bromo-L-tyrosine). Similar improvement can be achieved by exposing crystalline KBrO₃ for some time to CF₃^82mBr (or CF₃^80mBr) and dissolving subsequently the KBrO₃ in an acidic solution of the substrate. The radiochemical yields of ⁸⁰Br-5-bromodeoxyuridine and ⁸⁰Br-bromoacetic acid obtained by employing various modifications of the direct decay induced ⁸⁰Br labelling method which exposes substrate molecules to gaseous CF₃^80mBr are reported. The results indicate a drastic improvement of the amount of Br incorporated into these products if the labelling is accomplished by applying the “CF₃^80mBr-KBrO₃” gas exposure technique to induce a ⁸⁰Br for I exchange in the corresponding (inactive) iodo derivatives. The effect of several experimental conditions, such as the labelling time, the pH of the labelling medium, and the concentration of the substrate solutions, on the efficiency of the ⁸⁰Br incorporation via the “CF₃^80mBr-KBrO₃” gas exposure method resulting in carrier free radiobrominated compounds was investigated by using two model compounds, L-tyrosine and guanosine. The ⁸⁰Br labelling proceeds very rapidly in the case of the L-tyrosine and is fairly independent of “labelling time" and substrate concentration as long as a certain minimum amount of L-tyrosine is used. This is in contrast to the guanosine system where rapid secondary reactions reduce the initially high yields of ⁸⁰Br-guanosine at extended "labelling times” and where larger amounts of substrate are needed to produce optium yields. The chemical stability of ⁸⁰Br-guanosine and ⁸⁰Br-tyrosine was established by studying the dependence of the radiochemical yield on “labelling time” ( CF₃^80mBr-KBrO₃ gas exposure method ) and “post-labelling time” ( CF₃^80mBr-Cl₂ gas exposure method ). A radiochemical yield-time index is proposed to summarize the radiochemical yield dependence on “labelling” and “post-labelling” times in these two modified gas exposure labellings. It is hoped that this index will be helpful in making a priori judgements on the applicability of these labelled biomolecules for nuclear medical studies. Some preliminary mechanistic studies were carried out by using different reactants and model compound aniline. The proposed mechanism for both the direct excitation labelling and the CF₃^80mBr-KBrO₃ gas exposure method is electrophilic. / Ph. D.