A simple glass coulometer with mercury electrodes was designed to measure the amount of gas evolved upon passing current through single crystals, to determine the extent to which electrical conduction in these crystals is due to protons or other gas-forming ions. It is useful if at least one coulomb can be passed through the crystal in a reasonable time interval. With single crystals of KH₂PO₄ and Li(N₂H₅)SO₄ in this coulometer, the ratio of hydrogen evolution rate to crystal current indicates entirely protonic conduction.