  We have measured the effect of oxygen isotope substitution on the ab-plane
reflectance of underdoped YBCO. The frequency shift of the transverse optic
phonons due to the substitution of O-16 by O-18 yields an isotope effect of the
expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1.
The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of
alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the
superconducting state. These observations suggest that the shoulder is of
electronic origin and not due to a phonon mode as has been suggested recently.
