Little is known about the determining influences on the length of silent intervals at Intonational Phrase (IP) boundaries and no current models accurately predict their duration. A difficulty which faces the attempts to model pauses quantitatively is the high degree of variability exhibited by speakers in pause placement and duration. Using the paradigm of Synchronous Speech (speech elicited when asking two people to read a given text together) as a means to reduce inter-speaker variability, the present study investigates if pause duration is correlated with the length of sentences or phrases preceding and following a pause. The possible contribution of the length of surrounding phrases to pause duration at boundaries of different strengths is examined. Phrase-edge and phrase-medial durational processes and their correlation with pause duration are also analyzed in this investigation. The results presented in this study shed some light on the determining influences on pause duration, which might be important for better understanding how phrase-level prosodic structure is generated.