This article examines the adjectival and adverbial expressions it is certain/certainly, it is clear/clearly and it is obvious/obviously in a corpus of present-day English, the British National Corpus. It aims to answer two questions. One is the question of how the constructional properties of the adjectival expressions create specific behavioural properties which are different from those of the adverbs. The second question is how the adjectival expressions are used rhetorically and how their discourse functionality differs from that of the corresponding adverbial expressions. Starting out from Nuyts’ (2001) criteria, ‘intersubjectivity’, ‘salience’, ‘performativity’ and ‘discourse strategy’ , the paper examines the extent to which the data confirm the relevance of these factors. It is shown that the reasons why speakers opt for adjectival or adverbial expressions are complex and that an explanation cannot limit itself to a single factor.