  In a recent letter [1] Pilgram, Rice and Sigrist (PRS) propose to perform
oxygen isotope dependent conductivity measurements along the c-axis of the
cuprate superconductor BSSCO. In Raman isotope data for vibrations along the
c-axis from 1988 [2], and in site selective susceptibility experiments
determining Tc, no isotope effects were observed for oxygen ions in the
insulating layer [3]. Based on these earlier not by PRS quoted experiments, we
argue that the suggestion made in Ref. 1 is not worth to be undertaken, and if
it is carried through, no effect will be seen. From the meanwhile 12 years old
site selective data [3] the observed isotope effects stem to nearly 100% from
oxygen ions in the CuO2-planes. These results evidence polaronic behavior in
these planes [4].
