Automatic tagging in Spanish has historically faced many problems because of some specific grammatical constructions. One of these traditional pitfalls is the “se” particle. This particle is a multifunctional and polysemous word used in many different contexts. Many taggers do not distinguish the possible uses of “se” and thus provide poor results at this point. In tune with the philosophy of free software, we have taken a free annotation tool as a basis, we have improved and enhanced its behaviour by adding new rules at different levels and by modifying certain parts in the code to allow for its possible implementation in other EAGLES-compliant tools. In this paper, we present the analysis carried out with different annotators for selecting the tool, the results obtained in all cases as well as the improvements added and the advantages of the modified tagger.