We have studied the effect of isotopic substitution on the superconducting Tc in the 90-K superconductors Ba2YCu307 and Ba2EuCu307 by replacing ^^O with the heavier isotope '^O. Samples with approximately 75% of the '^O replaced by '^O were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected ~4%. The transition temperatures, however, are found to change by less than 0.2%. This change in Tc is much less than that expected for strongly coupled phonon-mediated superconductivity.