The aim of this study is to identify the relation between the interpretation of epistemic parentheticals in discourse and their prosodic realisation. Data drawn from a corpus of British English speech suggests that epistemic parentheticals (comment clauses such as I think, I believe) convey a spectrum of meaning from propositional to interpersonal. They have long been categorised simply as sentence adverbials with a meaning that relates to the truth value of the proposition. However, a study of their prosodic realisation suggests that they occupy a transitional place in the process of semantic change. They can express a wide range of meanings from propositional (sentential) meaning, through discourse meaning to the status of verbal filler. The analysis draws on theories of discourse, historical change and prosody. It makes an important contribution to the understanding of how prosody conveys apparently subtle shades of meaning that are nonetheless crucial for utterance interpretation, including degrees of speaker certainty, the identification of disfluency and the expression of politeness.