1. A method for performing thematic part-of-speech tagging for collocations having content-word pairs in a natural language text processing system comprising the steps of: identifying collocations of content-word pairs in a large corpus of text; calculating, for each of said collocation content-word pair identified, a variability factor which is a measure of variability of said collocation content-word pairs occurring in said text; storing said collocation content word pairs and associated variability factors in a collocation database; and using said database to tag collocation content-word pairs according to said variability factors, wherein collocation content-word pairs with high variability factors are tagged as having a verb and a noun thereat and collocation content-word pairs with low variability factors are tagged as having an adjective and a noun thereat or a noun and noun thereat.