We present measurements from the Pierre Auger Observatory related to
mass composition of ultra-high energy cosmic rays.
Using the fluorescence telescopes of the Observatory we determine the
distribution of shower maxima (Xmax) from 10^17.2 to 10^19.6 eV and
derive estimates of the mean and variance of the average logarithmic
mass of cosmic rays. The fraction of p, He, N and Fe nuclei as a
function of energy is derived by fitting the Xmax distribution with
templates from air shower simulations using the most recent version of
LHC-tuned hadronic interaction models.
Furthermore, we will discuss the analysis of the time structure of the
signals from air showers recorded with the water-Cherenkov detectors
to study the mass composition from 10^17.5 to 10^20 eV.