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.
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