[en] The understanding of the nature of ultra-high energy cosmic rays is one of the most intriguing open questions for current and future observatories. With its hybrid design and huge exposure, the Pierre Auger Observatory provides valuable statistical measurements of the chemical composition of cosmic rays with energies above 10¹⁷ eV, including the search for neutral primaries such as neutrinos and photons. We report on the most recent results which are based on the accurate measurement of the depth of the shower maximum, X_max, by the fluorescence telescopes and on the shape of the signals recorded by the water-Cherenkov detectors. The interpretation of these results in terms of mass composition is also discussed related to the hadronic interaction models used to describe the development of air showers. (paper)