This study examined property conflicts in thirty-two 20- and 30-month-old peer dyads during eighteen 40-min play sessions. Ownership influenced conflicts. Both 20- and 30-month-old owners claimed ownership (“mine”) and instigated and won property conflicts more often than non-owners. At 30 months, owners also resisted peers’ instigations more often than non-owners. Mothers’ interventions supported non-owners more often than owners, in part because owners initiated conflict more frequently. Children who received mothers’ support tended to win disputes. Finally, mothers’ support of owners and children’s adherence to ownership rights led to decreased conflict as relationships developed, supporting predictions based on theories concerning the social utility of ownership rights.