It is shown that radiation pressure acting on the material in the outer envelopes of QSOs will cause mass outflow unless the mass M of the QSO exceeds a critical value M5. If the QSOs are at cosmological distances, Ms is of order lOi M0, and otherwise scales according to intrinsic luminosity. Radiationpressure-driven mass loss could be responsible for the absorption4ine spectra observed in many QSOs If so, the presence of blueshifted absorption lines in some but not all QSOs indicates that typically M is of the same order as M5. It is also suggested that under radiative acceleration certain ratios of emissionline to absorption-line redshifts may be preferred as a result of a "line-locking" mechanism. Inferences are made concerning the magnitude of the gravitational component of the redshift. Possible tests of various aspects of the model are discussed.