Pulsed XeCl excimer laser ablation (lambda = 308 nm; irradiance approximate to 10 MW cm(-2)) of hydrated lanthanide oxalates into vacuum has been found to generate significant yields of lanthanide oxide cluster ions, the compositions of which were analyzed by reflectron time-of-night mass spectrometry. Cluster ions, Ln(m)O(n)(+) were identified for all of the lanthanides studied : Ln = Sm, Eu, Gd, Tb, and Yb. In each case, ions with metal contents (m) of up to at least 26 were detected, with the largest cluster identified being Tb63O94+ (mass = 11 516 amu). A distinctive sequence of cluster compositions was found for each Ln, apparently reflecting the relative accessibilities of the +2, +3, and +4 oxidation states. On the basis of the observed cluster stoichiometry distributions, it was concluded that trivalent metal sites were dominant in each series of Ln(m)O(n)+. However, divalent Eu sites were ubiquitous, and tetravalent Tb sites were also assigned. In addition, more metal-rich cluster ions were identified for Sm and Gd. A double-chain polymeric lanthanide oxide schematic structure is postulated to provide a coherent picture of the observed clustering behaviors.