The work in this thesis is concerned with some specific aspects of fission product chemistry encountered in a severe reactor accident. The fission products selected for this study were iodine, caesium and tellurium. Considerable emphasis is also placed on the involvement of structural materials such as boric acid in determining the fate of the fission products. Matrix isolation infrared spectroscopy and mass spectrometry were the techniques chosen to investigate the chemistry. The vapour above orthoboric acid has been studied by both mass spectrometry and matrix isolation infrared spectroscopy and found to consist of monomeric H3BO3. As a result of extensive isotope labelling it is possible to verify that molecular orthoboric acid has C3fc symmetry. Estimates of the inactive frequencies have been made by a partial normal co- ordinate analysis. The reaction of boric acid with caesium iodide has been investigated and it is shown that the volatile iodine species produced is hydrogen iodide. A semi-quantitative study has shown that between 1% and 10% of the available iodine is transported as hydrogen iodide and that under some circumstances the yield may be quantitative. Aspects of the fission product chemistry of tellurium under both reducing and oxidising steam conditions are considered. A matrix infrared study of hydrogen telluride and it deuterated analogues has been carried out. From this study the stretching frequencies have been resolved and assigned for the first time. The co-deposition of tellurium dioxide with potassium in nitrogen matrices has resulted in the identification of the new KTeO2 species. Also included, as an example of how matrix isolation and mass spectrometry can be used to investigate new high temperature species, is the identification of caesium thiotungstate, Cs2WS4, as the first ternary metal sulphide to be stable in the vapour phase.