The unique structural and electronic properties of graphene have great potential for the development of opto-electronic applications, although significant further studies and optimization are required. In such studies, to minimize parasitic doping effects caused by uncontrolled material adsorption, graphene is often investigated under vacuum. However, we have shown that an entirely unexpected strong n-doping of graphene due to chemical species generated by common ion high-vacuum gauges occurs in such systems. The effect - reversible upon re-
exposing the graphene to air - is significant, as doping rates can largely exceed 1012 /cm·hour, depending on pressure and the relative position of the gauge and the graphene device.