Abstract: A ring binder mechanism that retains loose-leaf pages and has ring members that readily lock together, preventing accidental loss of pages. The mechanism comprises a housing that supports two hinge plates for loose pivoting motion, bringing the ring members to either an open position or a closed position. The mechanism further comprises a control structure, which includes an actuating lever pivotally mounted on the housing. The actuating lever moves a travel bar and its locking elements for controlling the pivoting motion of the hinge plates. When the ring members are closed, the locking elements block the hinge plates from pivoting to open the ring members. In addition, a spring is attached to the travel bar for automatically biasing the travel bar and locking elements to the locked position when the ring members close.