[en] 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 Stueckelberg 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 G₄ 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.