When we compare corpora compiled at different periods we can notice that the overall frequency of the adverbs of certainty is different. For example, certainly and surely do not have such a strong position in the Bergen Corpus of London Teenage Language (COLT) as in the London-Lund Corpus (LLC). On the other hand, obviously and definitely are proportionately more frequent in the COLT corpus than in the LLC. In particular they have become more frequent in uses involving intensification and affective meaning. Changes in the language are intimately connected with grammaticalization. The article argues that the grammaticalization of obviously and definitely can be explained with reference to the characteristics of adolescent speech.