The aim of this research is to observe the usage of a semi-structured communication interface in collaborative problem-solving in the light of recent research on collaboration. Ten mixed pairs have to set up the schedule of a working conference by taking several constraints into account. The subjects can act on a shared problem representation and communicate synchronously by two modes inspired by previous work on structured communication interfaces. In the 'free' mode, they can type a message in a bare text field. In the 'structured' mode, utterances are issued either by pressing a button or by filling in text fields preceded by sentence openers. Data was automatically collected by a MOO[1] which served as backend for a graphical user interface written in JAVA. The results show that pairs who use the 'free' communication mode more than the 'structured' mode, produce more 'off-task' statements than the pairs who prefer the 'structured' mode. Another finding is that when strategic issues are discussed during the first third of total time, there is a tendency to be more accurate in placing the events at the right place at first trial.