We report infrared (IR) reflectivity measurements of single crystal YBa2Cu3O6, the semiconducting version of YBa2Cu3O7-δ (i.e., δ = 1). The near normal incidence abplane reflectivity clearly exhibits five of the six allowed Eu modes. Based on normal modes in K2NiF4-type materials, we offer a possible explanation for the unobserved Eu mode. Combining the single crystal data with our previously reported data on isomorphically and isotopically (18O) substituted ceramic YBa2Cu3O6, we determine the A2u modes. Thus, a fairly complete picture of the IR active phonons is obtained. The phonon assignments are discussed. © 1989.