  We generalize the recent proposal that invariance under T-duality leads to
additional non-geometric fluxes required so that superpotentials in type IIA
and type IIB orientifolds match. We show that invariance under type IIB
S-duality requires the introduction of a new set of fluxes leading to further
superpotential terms. We find new classes of N=1 supersymmetric Minkowski vacua
based on type IIB toroidal orientifolds in which not only dilaton and complex
moduli but also Kahler moduli are fixed. The chains of dualities relating type
II orientifolds to heterotic and M-theory compactifications suggests the
existence of yet further flux degrees of freedom. Restricting to a particular
type IIA/IIB or heterotic compactification only some of these degrees of
freedom have a simple perturbative and/or geometric interpretation.
