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Nitrite is part of many commercially available inhibitor formulations used for corrosion protection in water or water/glycol based heat transfer media in contact with carbon steel. In a heat transfer system of the OMV-AG used to heat crude oil to lower its viscosity, the nitrite concentration was found to decrease in about three months from 300 ppm to 10 ppm. The system is operated at a maximum temperature of 110 8C and 6 bar and an ethylene glycol/water mixture is used as the heat transfer media. An oxidation to nitrate was assumed, but the nitrate concentration was found to be constant. An analysis of the vapor phase present in the system showed next to significant amounts of hydrogen dinitrogen oxide. A possible microbiological infection of the system was investigated, but no bacteria were found. To find the reason for the reduction of nitrite, a laboratory investigation was performed. Stability of nitrite at elevated temperatures and pressure was tested in water and water/ glycol mixtures. It was found that nitrite is instable in glycol solutions at temperatures above 120 8C. The degradation reaction in aged and unused MEG/water mixtures was compared. An accelerated reduction reaction was found in aged heat transfer media.