  A sample of 198 soft X--ray selected active galactic nuclei (AGN) from the
ROSAT International X--ray Optical Survey (RIXOS), is used to investigate the
X--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14
erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2
of sky, and is almost completely identified; it is used in combination with the
Einstein Extended Medium Sensitivity Survey (EMSS), to give a total sample of
over 600 AGN. We find the evolution of AGN with redshift to be consistent with
pure luminosity evolution (PLE) models in which the rate of evolution slows
markedly or stops at high redshifts z>1.8. We find that this result is not
affected by the inclusion, or exclusion, of narrow emission line galaxies at
low redshift in the RIXOS and EMSS samples, and is insensitive to uncertainties
in the conversion between flux values measured with ROSAT and Einstein. We
confirm, using a model independent Ve/Va test, that our survey is consistent
with no evolution at high redshifts.
