A ring binder mechanism for retaining loose-leaf pages securely holds closed ring members together to prevent inadvertent loss of pages retained by the mechanism. The mechanism comprises a housing supporting two hinge plates for pivoting motion to open and close paired ring members mounted on the plates. Free ends of the paired ring members have interlocking configurations that securely join together when the ring members close. The configurations prevent misalignment of the closed ring members in all directions transverse to longitudinal centerlines of the ring members. The mechanism also includes locking elements that interact with the hinge plates to block their pivoting motion when the ring members are closed. This holds the free ends of the closed paired ring members together so that the interlocking configurations of the ring members remain interengaged.