The c-axis polarized phonon spectra of single crystals of YBa2Cu3O6+x, were measured for the doping range x = 0.5 → 0.95, between 10 and 300 K. The low background electronic conductivity, determined by Kramers–Kronig analysis of the reflectance, leads to a rich phonon structure. With decreased doping the five normally-active B1u modes broaden and the high-frequency apical-oxygen mode splits into two components. We associate the higher of these with the two-fold coordinated copper "sticks". The 155 cm−1 low-frequency mode, which involves the apical and the chain oxygens, splits into at least three components with decreasing doping. Some phonon anomalies that occur near Tc in the highly doped material occur well above Tc in the oxygen-reduced systems. An unusual broad phonon band develops in the normal state at ≈ 400 cm−1, which becomes more intense at low doping and low temperatures, borrowing oscillator strength from apical- and plane-oxygen modes resulting in a major transformation of the phonon spectrum below ≈150 K.