We study the back-reaction of moduli fields on the inflaton potential in
generic models of F-term inflation. We derive the moduli corrections as a power
series in the ratio of Hubble scale and modulus mass. The general result is
illustrated with two examples, hybrid inflation and chaotic inflation. We find
that in both cases the decoupling of moduli dynamics and inflation requires
moduli masses close to the scale of grand unification. For smaller moduli
masses the CMB observables are strongly affected.Comment: 5 page