With a so-called sniffer probe the fraction of hydrogen during discharges with deuterium NBI heating was studied in the plasma edge of the stellarator W7-AS. As expected,the ratios were significantly lower than in discharges with hydrogen NBI heating butwere higher than those in discharges with only ECRH. An examination of the possible reasons for this increase concludes that it is not only partially caused by the direct influence of neutral injection but also depends on the energy content of the plasma determined by the heatingpower and the confinement time. Of course, the isotope ratio also depends on the starting conditions,i.e. the isotope ratio before the onset of NBI.