Reflectance studies on single crystals of a series of rare-earth (R=Pr, Nd, Sm, Gd) copper oxides have been made to determine phonon frequencies as a function of rare-earth mass. The increase in interatomic force constants with increasing rare-earth mass, which leads to a reduction in the lattice parameters, increases the vibrational frequencies. In addition, the change from the T-phase La2CuO4 to the T<-phase R2CuO4 crystal structure causes changes in the phonon frequencies that are due both to the changing lattice parameters and to the relocation of the apical oxygen atoms. © 1991 The American Physical Society.