Gas phase ion mobility measurements have been used to examine the geometries of silicon-doped carbon cluster cations, CnSi+ (n = 3-69). The observed isomers resemble those found for pure carbon clusters : linear chains, monocyclic, bicyclic, and polycyclic rings, graphite sheets, and fullerenes. Generally, the silicon atom substitutes for a carbon atom and has only a minor effect on the structure. The linear chain and monocyclic ring isomers coexist for CnSi+ clusters over a wider range than for pure carbon cluster cations. The results provide the first conclusive experimental evidence that Ca2n-1Si+ fullerenes with an even number of atoms are closed-cage structures with the silicon atom networked into the cage. For C2nSi+ fullerenes with an odd number of atoms the silicon atom appears to be exohedral.