  Recent studies have highlighted the potential of jet substructure techniques
to identify the hadronic decays of boosted heavy particles. These studies all
rely upon the assumption that the internal substructure of jets generated by
QCD radiation is well understood. In this article, this assumption is tested on
an inclusive sample of jets recorded with the ATLAS detector in 2010, which
corresponds to 35 pb^-1 of pp collisions delivered by the LHC at sqrt(s) = 7
TeV. In a subsample of events with single pp collisions, measurementes
corrected for detector efficiency and resolution are presented with full
systematic uncertainties. Jet invariant mass, kt splitting scales and
n-subjettiness variables are presented for anti-kt R = 1.0 jets and
Cambridge-Aachen R = 1.2 jets. Jet invariant-mass spectra for Cambridge-Aachen
R = 1.2 jets after a splitting and filtering procedure are also presented.
Leading-order parton-shower Monte Carlo predictions for these variables are
found to be broadly in agreement with data. The dependence of mean jet mass on
additional pp interactions is also explored.
