  This work is about the structure of the symbolic Rees algebra of the base
ideal of a Cremona map. We give sufficient conditions under which this algebra
has the "expected form" in some sense. The main theorem in this regard
seemingly covers all previous results on the subject so far. The proof relies
heavily on a criterion of birationality and the use of the so-called inversion
factor of a Cremona map. One adds a pretty long selection of examples of plane
and space Cremona maps tested against the conditions of the theorem, with
special emphasis on Cohen--Macaulay base ideals.
