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Thermoelectric power ͑S͒ and lattice parameter (a, b, c) measurements are used to determine the change in the carrier concentration ͑n͒ in both isotope exchanged and oxygen deficient samples of La 0.65 Ca 0.35 MnO 3Ϫ␦ . It is observed upon substitution of the 18 O isotope for 16 O that the Curie temperature T C decreases but no significant changes in S at 290 K and a, b, or c are seen. In contrast, for samples that are oxygen deficient, significant changes in S at 290 K and a, b, and c are observed, in addition to decreases in T C . It is found that the estimated ⌬n correlating to the observed ⌬S cannot account for the isotope-induced change in T C . Therefore it is concluded that any oxygen deficiencies introduced by the isotope exchange are negligible and do not account for the T C change observed.

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