In addition to the LOM standard and instructional design specifications, as well as domain specific indexing vocabularies, a structured indexing vocabulary for the more elementary learning objects is advisable in order to support retrieval tasks of developers. Furthermore, because semantic indexing is seen as a difficult task, three issues concerning consistency in indexing learning objects were empirically investigated: (1) the extent to which different indexers annotate in the same way; (2) the extent to which structure in value lists supports consistent indexing; and (3) different degrees of consistency in annotating various media types and attributes. The results show that a standardized value list does not necessarily lead to a consistent application to learning objects. Differences occur, especially for more abstract attributes and media types. Ontologies can contribute to a higher consistency in indexing and could improve retrieval by making concepts that are more abstract available. (Contains 8 tables.)