  We consider Type IIB orientifold models on Calabi-Yau spaces with three-form
G-flux turned on. These fluxes freeze some of the complex structure moduli and
the complex dilaton via an F-term scalar potential. By introducing pairs of
D9-anti-D9 branes with abelian magnetic fluxes it is possible to freeze also
some of the Kaehler moduli via a D-term potential. Moreover, such magnetic
fluxes in general lead to chiral fermions, which make them interesting for
string model-building. These issues are demonstrated in a simple toy model
based on a Z_2 x Z_2' orbifold.
