A new mechanical system and a convenient piezoelectric driven Kelvin probe for the measurement of the contact potential difference (CPD) under ultrahigh vacuum are described. Charging effects on CPD measures are discussed. The probe is small (2.2 mm diam) and allows cartography of the surface. The mechanical system is frictionless so the distance between the sample and the probe can be maintained constant and at a low value (≊20 &mgr;m). By elimination of charging effects, reliable and reproducible results can be obtained within ±10 mV. The system is tested on gold vicinal surfaces. ©1995 American Institute of Physics.