We determine the N = 1 low energy effective action for compactifications of type IIB string theory on compact Calabi–Yau orientifolds in the presence of background fluxes from a Kaluza–Klein reduction. The analysis is performed for Calabi–Yau threefolds which admit an isometric and holomorphic involution. We explicitly compute the Kähler potential, the superpotential and the gauge kinetic functions and check the consistency with N = 1 supergravity. We find a new class of no-scale Kähler potentials and show that their structure can be best understood in terms of a dual formulation where some of the chiral multiplets are replaced by linear multiplets. For O3- and O7-planes the scalar potential is expressed in terms of a superpotential while for O5- and O9-planes also a D-term and a massive linear multiplet can be present. The relation with the associated F-theory compactifications is briefly discussed.