Injected ion drift tube techniques, including ion mobility measurements and annealing and fragmentation studies, have been used to examine the isomers present for NbCn+ (n = 15-50) clusters. Isomers attributed to niobium-containing monocyclic and bicyclic rings, graphitic sheets, and metallofullerenes have been identified. Monocyclic rings, where the niobium atom appears to be either inserted into or bound to the outside of the ring, dominate for NbCn+ with n < 22. Isomers assigned to bicyclic rings are first observed and become dominant around NbC22+. Unlike the bicyclic rings for C-n(+) and LaCn+, the NbCn+ bicyclic rings do not anneal into monocyclic rings. They probably consist of two rings joined together by a niobium atom. An isomer attributed to NbCn+ graphitic sheets is present for n > 22 and becomes important for dusters with around 30 carbon atoms. Metallofullerenes are first observed for NbC28+ and become a major isomer for clusters with n > 31. Both endohedral metallofullerenes and networked metallofullerenes (where the metal atom is part of the cage) have been identified, For clusters with more than around 30 carbon atoms the NbCn+ bicyclic rings can be annealed into metallofullerenes and, for the smaller ones, metal-containing graphitic sheets. The isomers observed for NbCn+ are similar to those found for pure C-n(+) and LaCn+, but the niobium atom has a substantial effect on the properties and the abundances of the different isomers.