PurposeTraditional subject indexing and classification are considered infeasible in many digital collections. This paper seeks to investigate ways of enhancing social tagging via knowledge organization systems, with a view to improving the quality of tags for increased information discovery and retrieval performance.Design/methodology/approachEnhanced tagging interfaces were developed for exemplar online repositories, and trials were undertaken with author and reader groups to evaluate the effectiveness of tagging augmented with control vocabulary for subject indexing of papers in online repositories.FindingsThe results showed that using a knowledge organisation system to augment tagging does appear to increase the effectiveness of non‐specialist users (that is, without information science training) in subject indexing.Research limitations/implicationsWhile limited by the size and scope of the trials undertaken, these results do point to the usefulness of a mixed approach in supporting the subject indexing of online resources.Originality/valueThe value of this work is as a guide to future developments in the practical support for resource indexing in online repositories.