The objective of this report was to determine the effect of various types of indexing aids on the minimum reliability of indexers. The three types of tools tested as indexing aids on a collection of randomly selected chemical patents were a classificatory device (Manual of Classification of the U. S. patent Office), an alphabetical subject-authority list of terms (Chemical Patents Code List of Documentation Incorporated), and a concept-associative tool (Chemical Engineering Thesaurus of the American Institute of Chemical Engineers). The former two tools registered a highly significant improvement of the base zero inter-indexer consistency; the concept-association aid, on the other hand, failed to show any effect. The analyses and interpretation of the results indicate that an improvement in indexer reliability, and hence in the quality of indexing, can be brought about through the use of prescriptive, rather than suggestive, vocabularies which formalize the relationships among terms so as to invariable enjoin the indexer's assignment of index terms. Indexing aids which display numerous variable ill-defined relationships among terms appear to be acting in the opposite direction.