Careful measurements of rest potentials of platinum electrodes (Eh) in the H 2S-H 2O system were made together with measurements of the pH and the potentials of Ag-Ag 2S electrodes (E s 2-). The potentials of the platinum electrode are characteristic of H 2S chemisorption followed by H 2S discharge on the platinum. The relations between Eh, E s 2- and pH are characteristic of the H 2S-H 2O system and of the hydrogen sulfide concentration. Thus these parameters can be employed to distinguish H 2S-H 2O systems from H 2S-S 8-H 2O systems. Field measurements on hot sulfurous springs are in agreement with laboratory data.