O(2) evolution was observed upon mixing aqueous [(terpy)(H(2)O)Mn(O)(2)Mn(H(2)O)(terpy)](NO(3))(3) (1, terpy = 2,2':6',6″-terpyridine) with aqueous solutions of Ce(4+). However, when the solution of 1 was incubated at pH = 1 (by dissolving in dilute HNO(3)) before mixing with Ce(4+), very small amounts of O(2) were observed. This observation of acid-induced deactivation suggests an explanation, both for the previously reported lack of O(2) evolution from aqueous solutions of 1 with Ce(4+) as oxidant, and the present observation of low amounts of O(2) production with the very acidic Ce(4+) reagent. Evidence is provided for water being the source of evolved O(2), and for the requirement of a high valent multinuclear Mn species for O(2) evolution. We test the possibility of complications in the use of cerium ammonium nitrate (CAN) in oxidation chemistry due to the presence of the oxidizable NH(4) (+) ion.