The surface conductance of hydrogen-terminated diamond was investigated under air, HCl, and NH3 vapors using tungsten point contacts, gold/titanium ohmic contacts, and aluminum rectifying contacts. In all cases the conductance decreased with increasing pH. Upon imposition of a step temperature change, there is a transient increase in conductance followed by a gradual decrease to a steady state value. The transient conductance maxima increased with temperature. The steady state conductance decreased with increasing temperature, increased at higher humidity, and was quenched by organic liquids that wet the diamond surface. The results are in agreement with the electrochemical transfer-doping model. (C) 2004 The Electrochemical Society.