The object of this thesis was to examine microbial interactions in offshore crude oil storage systems with special reference to the role played by the sulphate-reducing bacteria (SRB). The aim was to characterise the SRB present in such systems; to isolate and characterise crude oil-degrading bacteria and to develop simple models of the system. SRB were shown to be present in samples from offshore by the detection of high levels of SRB and sulphide, and of depleted sulphate levels. SRB were enriched for, isolated and characterised from offshore oil storage facilities and pure cultures of the organisms <i>Desulfovibrio desulphuricans, Desulfovibrio sapovorans, Desulfobacter postgatei</i> and <i>Desulfobulbus propionicus</i> were isolated. <i>Desulfobacter</i> was shown to be the key organism in this environment, responsible for the terminal oxidation of acetate produced from the incomplete oxidation of higher fatty acids by the <i>Desulfovibrio</i> spp. and the <i>Desulfobulbus</i>. The enrichment, isolation and partial characterisation of oil-degrading bacteria of the genera <i>Pseudomonas</i> and <i>Micrococcus</i> was also carried out. These organisms were shown to use both aliphatic and aromatic components of crude oil. The breakdown of hydrocarbons provides both the anaerobic conditions and carbon sources necessary for the growth of SRB. Liquid and gel-stabilised model systems in which both the temporal and spatial development of oil-degrading communities (including SRB) were constructed.