Hydrogen-terminated diamond (HD) samples possess a p-type surface conductivity (SC) which is caused by transfer doping to an adsorbed liquid electrolyte layer. We report on gas sensing experiments on such samples and show that these selectively respond to analyte gases that can undergo electrolytic dissociation in the surface electrolyte layer. These gas sensing interactions occur at room temperature and are far more selective than sensing interactions at heated metal oxide layers. Successive substitution of surface hydrogen atoms by oxygen atoms causes the sensor baseline resistance and the gas-induced resistance changes to increase. This latter observation suggests that a small number of O-termination sites may have a catalytic effect on the gas sensing interactions. Increased temperature, O3 and UV light exposure all reduce the sensor recovery time constants. Heating beyond the water evaporation threshold (∼200 °C) causes the surface electrolyte layer to disappear and the gas sensing effect to vanish. Re-adsorption of the surface electrolyte layer re-establishes both the sensor baseline resistance and the gas sensing effect. A model for the dissociative gas response is proposed that accounts for the observed experimental facts.