This research study investigated the behaviour and interactions of university students as they engaged with ill-structured and well-structured assessment tasks. Twenty students participated in a one-semester intensive design study in a course on "Vietnamese traditional culture". The students worked in five-person groups that were mixed-gender and heterogeneous in terms of achievement. The results show that when the students engaged in ill-structured assessments, they were more co-operative, provided more helpful assistance to each other and perceived small-group work as more enjoyable than when they were engaged in well-structured assessments. These findings suggest that more open-ended assessment tasks increase productive learning behaviours in small groups and have a positive effect on the interaction of low-achieving students and their positive perceptions of group work. (Contains 4 tables.)