In this paper, we present an ontology-based methodology and architecture for the comparison, assessment, combination (and, to some extent, also contrastive evaluation) of the results of different linguistic tools. More specifically, we describe an experiment aiming at the improvement of the correctness of lemma tagging for Spanish. This improvement was achieved by means of the standardisation and combination of the results of three different linguistic annotation tools (Bitext's DataLexica, Connexor's FDG Parser and LACELL's POS tagger), using (1) ontologies, (2) a set of lemma tagging correction rules, determined empirically during the experiment, and (3) W3C standard languages, such as XML, RDF(S) and OWL. As we show in the results of the experiment, the interoperation of these tools by means of ontologies and the correction rules applied in the experiment improved significantly the quality of the resulting lemma tagging (when compared to the separate lemma tagging pe...