Abstract : The significant attenuation of the cosmic ray flux above ~5 × 10^19 eV suggests that the observed high energy spectrum is shaped by the so-called GZK effect (GZK: Greisen-Zatsepin-Kuzmin). This interaction of ultra-high energy cosmic rays (UHECRs) with the ambient radiation fields also affects their composition. We review the effect of photodissociation interactions on different nuclear species and analyze the phenomenology of secondary-proton production as a function of energy. We show that, by itself, the UHECR spectrum does not constrain the composition of cosmic rays at their extragalactic sources. While the propagated composition (i.e., as observed at Earth) cannot contain significant amounts of intermediate mass nuclei (say between He and Si), whatever the source composition, and while it is vastly proton dominated when protons are able to reach energies above 10^20 eV at the source, we show that the propagated composition can be dominated by Fe and sub-Fe nuclei at the highest energies, either if the sources are very strongly enriched in Fe nuclei (a rather improbable situation), or if the accelerated protons have a maximum energy of a few 10^19 eV at the sources. We also show that in the latter cases, the expected flux above 3 × 10^20 eV is very much reduced as compared to the case when protons dominate in this energy range, both at the sources and at Earth.