Studies of interactive systems in museums have raised important design considerations, but so far have failed to address sufficiently the particularities of family interaction and co-operation. This paper introduces qualitative videobased observations of Japanese families using an interactive portable guide system in a museum. Results show how unexpected usage can occur through particularities of interaction between family members. The paper highlights the necessity to more fully consider familial relationships in HCI. Author Keywords Museums, conversation analysis, video analysis, actornetworks, families. ACM Classification Keywords H5.m. Information interfaces and presentation (e.g., HCI): Miscellaneous.