Long-term experiments (84–126 days long) on biochemical oxygen demand (BOD) were conducted to study the BOD kinetics in water samples from more than 30 waterbodies of the Karelian region and from the White Sea in different seasons of 2012–2017.The experiments were conducted under two incubation temperatures (10 and 20 °C or 2 and 10 °C) to estimate the temperature coefficient. Two approaches were applied to describe the BOD kinetics. The first one originated from a compilation of the data previously obtained for natural, contaminated and waste waters. The second one was based on visual classification involving BOD curves plotted for different waterbodies of Karelia. The distinctive feature of the long-term BOD test conducted here is that the BOD curve developed by the “apparent exponent” (APE) model during 30–80 days from the beginning of the experiments in some studied cases. This situation is completely different from the classical case, where autochthonous organic matter oxidation follows the exponential model in the first 15–20 days. BOD curve kinetic parameters were compared for еру APE and exponential models. The origins of APE curve formation were identified and described. The suspension of BOD development under various temperature conditions was explained. In the future, long-term BOD tests can be applied for water quality assessment together with the characteristics of organic matter content.