Damage to buried structures caused by the oxidation of pyrite in structural backfill and the surrounding natural ground affected a number of British highway schemes during the late 1990s. Although the levels of sulphate in these materials had been determined prior to construction by the methods current at that time, the potential for sulphate that was subsequently generated by the oxidation of pyrite was not adequately assessed. Accordingly, new test methods, guidance and appraisal strategies were developed for assessing structural backfills and soil stabilization materials and potentially aggressive ground. This included defining a series of parameters that were then used in the classification and assessment of the natural ground and geomaterials in contact with structures. The experience of the authors has indicated that many practitioners do not to fully appreciate the chemical origin of the sulphur species or the precise meaning of the terminology, leading to misuse of the terminology. There is much scope for confusion and significant risk both that materials will be needlessly rejected and that inappropriate materials or construction practices will be used. The potential effects of recent changes to the test methods for sulphur compounds in the UK Specification for Highway Works provide additional justification for practitioners and researchers to understand what the tests measure and how to use the results. The aim of this paper is to provide a clear explanation of the sulphur chemistry and terminology for use in ground engineering.