  We discuss the role of Abelian gauge symmetries in type IIB orientifold
compactifications and their F-theory uplift. Particular emphasis is placed on
U(1)s which become massive through the geometric St\"uckelberg mechanism in
type IIB. We present a proposal on how to take such geometrically massive U(1)s
and the associated fluxes into account in the Kaluza-Klein reduction of
F-theory with the help of non-harmonic forms. Evidence for this proposal is
obtained by working out the F-theory effective action including such
non-harmonic forms and matching the results with the known type IIB
expressions. We furthermore discuss how world-volume fluxes on D3-brane
instantons affect the instanton charge with respect to U(1) gauge symmetries
and the chiral zero mode spectrum. The classical partition function of
M5-instantons in F-theory is discussed and compared with the type IIB results
for D3-brane instantons. The type IIB match allows us to determine the correct
M5 partition function. Selection rules for the absence of chiral charged zero
modes on M5-instantons in backgrounds with G_4 flux are discussed and compared
with the type IIB results. The dimensional reduction of the democratic
formulation of M-theory is presented in the appendix. This article is based on
the author's PhD thesis but includes minor modifications.
