Syntactic disambiguation frequently requires knowledge of the semantic categories of nouns, especially in languages with free word order. For example, in Spanish the phrases pintó un cuadro un pintor (lit. painted a picture a painter) and pintó un pintor un cuadro (lit. painted a painter a picture) mean the same: ‘a painter painted a picture’. The only way to tell the subject from the object is by knowing that pintor ‘painter’ is a causal agent and cuadro is a thing. We present a method for extracting semantic information of this kind from existing machine-readable human-oriented explanatory dictionaries. Application of this procedure to two different human-oriented Spanish dictionaries gives additional information as compared with using solely Spanish EuroWordNet. In addition, we show the results of an experiment conducted to evaluate the similarity of word classifications using this method.