Mobilities of CO + 2 and N 2 O + ions in Ne gas were measured against E / N using an injected-ion drift tube mass spectrometer. It was found that the reduced mobilities of both ions almost coincide with each other. They are constant at low E / N , and decrease monotonously above 30Td. The zero field reduced mobilities are measured as 7.20 cm 2 /V-sec for both ions, which is about 20% larger than the polarization limit of 5.83 cm 2 /V-sec.