Backward-wave phonon echoes (polarization echoes), heretofore only observed in piezoelectric crystals, were generated in several different glasses. Echo amplitudes were found to be proportional to the number of unassociated hydroxyl ions in the samples. It is proposed that OH- ions in glass form a distribution of inhomogeneously broadened resonances, sensitive to both ultrasonics and electromagnetic fields. Effect of this conjecture on other measurements in glasses are discussed, as are various measured properties of the echoes. © 1977 The American Physical Society.