  Stabilization of closed string moduli in toroidal orientifold
compactifications of type IIB string theory are studied using constant internal
magnetic fields on D-branes and 3-form fluxes that preserve N=1 supersymmetry
in four dimensions. Our analysis corrects and extends previous work by us, and
indicates that charged scalar VEV's need to be turned on, in addition to the
fluxes, in order to construct a consistent supersymmetric model. As an explicit
example, we first show the stabilization of all Kahler class and complex
structure moduli by turning on magnetic fluxes on different sets of D9-branes
that wrap the internal space T^6 in a compactified type I string theory, when a
charged scalar on one of these branes acquires a non-zero VEV. The latter can
also be determined by adding extra magnetized branes, as we demonstrate in a
subsequent example. In a different model with magnetized D7-branes, in a IIB
orientifold on T^6/Z_2, we show the stabilization of all the closed string
moduli, including the axion-dilaton at weak string coupling g_s, by turning on
appropriate closed string 3-form fluxes.
