Controlled vocabularies prove useful for improving the quality of topical keywords describing SSH publications. This presentation deals with the challenge of improving the quality of keywords which are expressed in the form of strings (keywords-strings). Experiences from Dariah.lab project (lab.dariah.pl) and TRIPLE project show that these keywords-strings on many occasions originate from controlled vocabularies if they were applied by institutional actors (e.g. data officers in digital libraries) and hence can be easily enriched with references to controlled vocabularies. On the other hand, keywords-strings from authors are less frequently originating from these kinds of resources. This presentation outlines automated and semi-automated methods for enriching both types of keywords-strings with references to controlled vocabularies.