  We calculate the dilution of the baryon-to-photon ratio by the decay of
superstring axions. We find that the dilution is of the order of $10^7$. We
review several models of baryogenesis and show that most of them can not
tolerate such a large dilution. In particular, only one current model of
electroweak baryogenesis possibly survives. The Affleck-Dine mechanism in SUSY
GUTs is very robust and the dilution by axions could contribute to the dilution
required in these models. Baryogenesis scenarios involving topological defects
and black hole evaporation are also capable of producing a sufficiently large
baryon asymmetry.
