We use an ab initio B3LYP approximation to perform a quantitative analysis of the electronic and structural properties of hydrogen terminated (100), (110), and (111) diamond surfaces. We also study the charge transfer produced by the adsorption of a number of molecular species, including NO2, NO, and O3. Our results shed light on the understanding of hydrogenated diamond surface conductivity and demonstrate that combination of surface termination and molecular species is key to control the hole doping on diamond. This study, which does not rely on any empirical parameters, also provides guidelines for the choice of adsorbate molecules that yield desired hole injection to the hydrogenated diamond surfaces.