Experiments in iron-rich interstitial water taken from sediment of Linsley Pond, Connecticut have shown that a potential-determining reaction exists for platinum electrodes which do not carry a deposit of ferric oxide (as is generally required in experiments of this type). The stable redox potentials were more negative than predicted by the equation Eh=0.965-0.177pH-0.059 log a Fe ++ by about 100 mv. The oxide layer on the electrode, where present, disappeared slowly in this water as its potential drifted down from an initial value close to the ferrous ion-ferric oxide Nernst potential. Current-potential curves revealed several electroactive substances, none of which could be positively identified with Fe (super ++) ion.