The first polarized infrared-reflectivity measurements of untwinned crystals of YBa2Cu3O7-y as a function of y are reported. We separate the chain and plane contributions to the infrared conductivity, and find that for crystals with Tc50 K the prominent normal state midinfrared feature is associated with the chains. A threshold in the plane conductivity at 500 cm-1 has a strong temperature dependence just below Tc suggesting an association with the superconducting gap. Its temperature dependence above and below Tc is quite similar to that of relevant NMR data, and is suggestive of gap development in the normal state. © 1991 The American Physical Society.