Abstract. Spontaneous speech is produced and probably also perceived in some kinds of units. This paper applies the perceptually defined intonation units to segment spontaneous Mandarin data. The main aim is to examine spontaneous data to see if linguistic cues which mark the unit boundaries exist. If the production of spontaneous speech is a kind of concatenation of these "chunks", we can deepen our understanding of human language processing and the related knowledge about the boundary markings can be applied to improve language models used for automatic speech recognizers. Our results clearly show that discourse items and repair resumptions, which are typical phenomena in spontaneous speech, are mostly located at the boundary of intonation unit. Moreover, temporal marking of items at unit boundary is empirically identified through a series of analyses making use of segmentation of intonation units and measurements of syllable durations.