The reaction of Th(NO3)(4)center dot 5H(2)O with 3 equiv of 2,2',6',2"-terpyridine (terpy) in a mixture of acetonitrile and methanol results in formation of the trinuclear thorium peroxide cluster [Th(O-2)(terpy)-(NO3)(2)](3). This cluster is assembled via bridging by mu eta(2):eta(2) peroxide anions between thorium centers. It decomposes upon removal from the mother liquor to yield Th(terpy)(NO3)(4) and Th(terpy)(NO3)(4)(EtOH). The peroxide formation appears to be radiolytic in origin and is, most likely, generated from radiolysis of water by short-lived daughters generated from Th-232 decay. This cluster does not form when freshly recrystallized Th(NO3)(4)center dot 5H(2)O is used as the starting material and requires an aged source of thorium. Analysis of the bonding in these clusters shows that, unlike uranium(VI) peroxide interactions, thorium(IV) complexation by peroxide is quite weak and largely ionic. This explains its much lower stability, which is more comparable to that observed in similar zirconium(IV) peroxide clusters.