A mathematical model for oxygen fluctuations in a river reach, involving photosynthesis, total community respiration, and reaeration is presented. Analogy is established between diurnal oxygen fluctuations and diurnal pH-fluctuations. Comparison is made between oxygen production and respiration, and carbon dioxide consumption and production. Twin curve analysis of diurnal oxygen fluctuations and diurnal pH-fluctuations is made of measurements on a single river reach in Denmark. The measurements were carried out about once a month for a little more than one year. The analysis has revealed seasonal variations of the river parameters: total daily gross production of oxygen, total community respiration and reaeration coefficient. These parameters are important for river modelling.