We report on the operation of ungated surface conductive diamond devices in electrolytic solutions. The effect of electrolyte pH on the channel conductivity is studied in detail. It is shown that fully hydrogen terminated diamond surfaces are not pH sensitive. However, a pronounced pH sensitivity arises after a mild surface oxidation by ozone. We propose that charged ions from the electrolyte adsorbed on the oxidized surface regions induce a lateral electrostatic modulation of the conductive hole accumulation layer on the surface. In contrast, charged ions are not expected to be adsorbed on the hydrogen terminated surface, either due to the screening induced by a dense layer of strongly adsorbed counter-ions or by the absence of the proper reactive surface groups. Therefore, the modulation of the surface conductivity is generated by the oxidized regions, which are described as microscopic chemical in-plane gates. The pH sensitivity mechanism proposed here differs qualitatively from the one used to explain the behavior of conventional ion sensitive field effect transistors, resulting in a pH sensitivity higher than the Nernstian limit.