Polycrystalline diamond films have been found to displayp-type surface conductivity. No bulk impurity is added to the films; thep-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range1017–1019 cm−3have been measured; control over the carrier concentration can be achieved by annealing the “as-grown” films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been explored and a value of>70 cm2/Vshas been measured for a film with a carrier concentration of∼5×1017 cm−3,the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond. ©1998 American Institute of Physics.