Abstract A continuously recording flow respirometer based on an oxygen electrode is described. It was designed to give long-term respiration measurements without loss of the carbonate-bicarbonate buffering character of sea water, and without the build up of waste products as in closed systems. An experiment on the microbial degradation of oil is described, together with investigations of a photochemical oxidation discovered fortuitously during similar experiments. The relationship between nutrient supply and bio-oxidation rate is discussed.