We have investigated a series of samples embedding ZnO/(Zn,Mg)O quantum wells of different sizes, in wurtzite phase, by using time-resolved photo luminescence. The samples were grown by molecular beam epitaxy on ZnO templates, themselves deposited on sapphire substrates. The presence of large internal electric fields in these quantum wells manifests itself not only through the energies of the optical recombinations, but also through the size dependence of the recombination times. An envelope-function model that includes the variational calculation of the exciton binding energy allows us to determine a value of 0.9 MV/cm for the internal electric field. (c) 2005 Elsevier B.V. All rights reserved.