The aim of this study was to determine the effect of abiotic aging of biochars under controlled laboratory conditions on its physicochemical properties and in consequence on their stability. Biochars (BCs) produced at 500 and 700 °C from willow or sewage sludge were incubated at different temperatures (−20, 4, 20, 60, or 90 °C) for 6 and 12 months. Pristine (i.e. immediately after their production) and aged BCs were characterized using a range of complementary methods. As a result of simulated temperature aging, there was a change in all biochar properties studied, with the direction of these changes being determined by both the type of feedstock and biochar production temperature. At all temperatures, aging was the most intense during the first 6 months and led to oxidation of the biochars and removal of the most labile components from them. The intensity of these processes increased with increasing aging temperature. Incubation of the biochars for another 6 months did not have such a significant effect on the biochar properties as that observed during the first months of incubation, which is evidence that the biochars had reached stability. The sewage sludge-derived biochars with a higher mineral content than the willow-derived biochars were less stable. The low-temperature biochars (BC-500) with lower aromaticity were more prone to abiotic oxidation than the high-temperature biochars (BC-700) with higher aromaticity and structurally ordered C. Based on this study, it can be concluded that aging induced changes will be specific for each biochar, i.e. they will depend on both the type of feedstock and pyrolysis temperature. Nonetheless, all biochars will be oxidized to a smaller or greater extent, which will result in an increase in the number of surface oxygen functional groups, an increased degree of their hydrophilicity and polarity, and a decrease in pH.