  While stereo measurements of extensive air showers allow a more precise
determination of the depth of shower maximum and hence the composition of
UHECR's, monocular measurements allow one to go much lower in energy. Since the
composition of UHECR seems to constant throughout the HiRes stereo energy range
but changing just below it, this is not a trivial lowering of the energy
threshold. We fit the observed Xmax distribution to a combination of expected
proton and iron Xmax distributions, using two different interaction models, to
determine the relative fraction of light and heavy components throughout the
HiRes monocular energy range. Using a two component fit allows both the mean
Xmax and the width of the Xmax distribution to contribute composition
measurement and allows us to deal with the Xmax acceptance bias caused by
limited elevation coverage. An updated analysis from a larger data set will be
presented in Pune.

Astrophysics