  The z ~ 0.701 absorption system towards the QSO PG1718+4807 is the only
example of a QSO absorption system which might have a deuterium/hydrogen ratio
approximately ten times the value found towards other QSOs. We have obtained
new STIS spectra from the Hubble Space Telescope of the Lyman alpha and Lyman
limit regions of the system. These spectra give the redshift and velocity
dispersion of the neutral hydrogen which produces most of the observed
absorption. The Lyman alpha line is too narrow to account for all of the
observed absorption. It was previously known that extra absorption is needed on
the blue side of the main H I near to the expected position of deuterium. The
current data suggests with a 98% confidence level that the extra absorption is
not deuterium. Some uncertainty persists because we have a low signal to noise
ratio and the extra absorption - be it deuterium or hydrogen - is heavily
blended with the Lyman alpha absorption from the main hydrogen component.
