Ultraviolet photoelectron spectra (UPS) of three Tm@C-82 isomers were measured with a synchrotron radiation light source. The upper valence band spectra (0-5 eV) of three isomers are different each other and they also differ from those of other trivalent metal atoms encapsulated metallofullerenes, such as La@C-82, Sc@C-82 and Gd@C-82. Resemblance of the spectra of Tm@C-82 (II) and Ca@C-82 (III) suggests identical cage structures of these two metallofullerenes. The UPS are well reproduced by simulated spectra obtained with ab initio calculation, which indicates that isomers I-III have C-s (No. 4), C-2 (No. 5) and C-2v (No. 9) cage structures. (C) 2004 Published by Elsevier B.V.