  We investigate the effects of dynamical Comptonization on the emergent
radiation spectrum produced by near critical accretion onto a neutron star. The
flow dynamics and the transfer of radiation are self--consistently solved in
the case of a spherically symmetric, ``cold'', pure scattering flow, including
general relativity. A sequence of models, each characterized by the value of
the total observed luminosity, was obtained assuming that the spectrum at the
star surface is black body in shape. It is found that when the luminosity
approaches the Eddington limit dynamical effects become important shifting the
spectrum to the blue and producing a power--law, high--energy tail. The
relevance of these results in connection with the observed spectral properties
of LMXBs, and of Cyg X--2 in particular, are discussed.
