The electron transfer from an H-terminated diamond (100)-2 x 1 surface to a neutral or acidic water adlayer has been theoretically investigated, using quantum mechanical DFT calculations under periodic boundary conditions. A surface conductivity of p-type was found to be induced by the acidic environment. An electron transfer of 1.8 electrons per surface unit cell was observed to take place from the upper part of the diamond valence band to the lowest unoccupied molecular level of the aqueous adlayer that contains one H3O+ ion. The result is a hole delocalized over the whole diamond model slab. Also, a pronounced weakening of the H3O+ bonds by the interaction with the diamond surface is observed.