Abstract: The acoustic characteristics and the perceived boundary strength of information unit boundaries in monologues are studied. The studied information units were clauses, sentences and paragraphs. The acoustic analysis was based on a prosodically labeled corpus of ten monologues. The main results were as follows:(a) There is a significant difference in the perceived degree of boundary strength for clauses (corresponding to intonational phrase), sentences (including simple sentences and complex sentences) and paragraphs. There is no significant difference in the perceived degree of the boundary strength at the edges of simple and complex sentences, nor is there significant difference in the acoustic variables measured (e.g. pitch reset and silence), suggesting that simple and complex sentences constitute a single type of prosodic constituent. (b) The pitch-based cue used to distinguish between different information units appears to be the difference between pre- and post-boundary syllables, but not simply the pitch of either syllable alone. (c) Declination occurs within intonational phrases, but not across or between larger units. (d) Silent pauses are longer and more varied between larger units. Moreover, there is a significant positive correlation between silent pause duration and the degree of pitch reset at clause boundary.