The combined effect of water adlayer composition and surface termination on diamond surface electrochemistry, has been studied theoretically using Density Functional Theory (DFT) calculations. The terminating species included H, O(ontop), O(bridge), OH and NH2. The chemical composition of the water adlayer was altered by using a very thin layer of water only, or by introducing oxygen, ozone or hydroxonium ions (H3O+) into the adlayer. A partial electron transfer toward the atmospheric adlayer was observed for the situation with either an H- or NH2-terminated diamond surface. Corresponding calculations for oxygen-termination (O(ontop) or O(bridge)), did not render any significant amount of electron transfer. The situation was completely different for the situation with OH-termination. The degree of electron transfer was approximately of the same order as for H- and NH2-terminations. The presence of oxidative species like oxygen, ozone, and H30+ (or combinations thereof) were observed to significantly increase the degree of electron transfer for the situation with either NH2-, OH-, or H-terminated diamond (100)-2 × 1 surfaces. Adsorption energy calculations revealed, with some exceptions, a quite good correlation between diamond//adlayer adhesion strength and degree of interfacial electron transfer. The electron transfer process were further verified and analyzed by performing partial density of state (pDOS) calculations for some selected diamond//adlayer systems.