  The effect of the 16O-->18O substitution on the superconducting transition
temperatures Tc of NaxCoO2.yH2O has been studied for several sets of samples.
Each set has two kinds of samples, one with the oxygen sites in the CoO2 layers
filled with 16O atoms and another with these sites partially substituted with
18O isotope. These two kinds of samples were prepared in parallel in whole
preparation processes. For all the sample sets, we have not found any
significant difference of the Tc values between the non-substituted and
substituted samples, which indicates that the oxygen isotope effect is not
appreciable in NaxCoO2.yH2O. The result, however, does not necessarily exclude
the possibility of the phonon mechanism of the superconductivity, because the
strong correlation effect suppresses the isotope effect as pointed out
previously.
