Arc formation at metal surfaces in a hydrogen plasma has been studied at gas pressures of 25-60 mtorr. The plasma was generated by means of a high current cold cathode Penning-type discharge with axial magnetic field, which defined the electrode areas receiving ion bombardment. Planar cathodes and probe wires of various metals were subjected to 100 μsec and 30 μsec pulses respectively, each at about 10 A cm^-2 and 2 kv. The occurrence of arcs was investigated over long periods, up to 10⁶ pulses at a rate of one per second.Measurements of time-lag to arc formation showed two distinct stages in the conditioning of the metals under ion bombardment. Initially, specimens with contaminated or oxidized surfaces arced at every pulse, with mean time-lags increasing from about 2 μsec to the full pulse length over a number of pulses; this number was dependent on the degree of contamination. Finally, when sputtering had removed surface impurities, the few arcs still occurring formed near the start of a pulse and occurred randomly with the number of pulses. The final arcing rate is shown to correlate with the impurity content of the metal. The effects of heat treatment in hydrogen before test and of heating during test were examined. For example, zero arcing rate was obtained over 5 × 10⁵ pulses with high purity platinum maintained at 700°C in hydrogen.Local heating of particles on the surface under ion bombardment is consistent with the initial arcing behaviour. Field emission associated with impurities within the metal may account for the arcing behaviour after conditioning.